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‫Requirements for this lecture are that you understand what an image is and that you've done a little

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‫bit of work with containers.

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‫In this lecture, we're going to take a quick tour around Docker Hub and get to know a few of its basic

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‫features. Then we'll kind of discuss the difference between an official image and maybe other just

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‫as good images, and how to tell the difference between a good image and a bad image.

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‫Then I'll show a little bit around downloading images and how you can see the different tags that

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‫they'll use and the difference between an Alpine image and other image options.

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‫...

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‫You should have signed into hub.docker.com. When you sign in your default home page is your

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‫dashboard, which will show you all of your repositories, public and private.

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‫You're not going to have any yet, but you can see I have some here.

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‫They're all public.

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‫I have one private. For now let's go to the top and search nginx. What we'll get back is the

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‫search results of Docker Hub and you'll see that there's a lot. I could scroll for pages and pages.

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‫One of the challenges is there's 15,000 images of Nginx.

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‫That's a lot.

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‫So I pretty much just use one or two.

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‫The official one right here,

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‫you'll see at the top, has 10 million-plus pulls.

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‫There's several characteristics to figuring out what's the right image.

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‫Like me, you're going to start using the images for your workflow that are related to what you do

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‫and you're going to get used to specific ones. You're going to like specific ones.

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‫Typically, we always start with the official.

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‫The official is going to be the only one with the word 'official' actually below it.

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‫It will also be the only one where its name doesn't have a forward slash in it.

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‫When you and I create images on Docker Hub, we have to create them with our account name in front

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‫of them.

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‫So when you look at all these other ones, these are account names slash image repository names.

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‫It might be actually an organization slash nginx or something like that.

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‫But the only ones that get to have just the name of the repo are considered official. Official images

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‫are ones that Docker, Inc. actually has a team of people that help take care of them, ensure that they

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‫have quality documentation, that they're well tested and that they're put together properly with Dockerfiles

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‫that are obeying best practice rules. They usually work with the official team of that software who

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‫actually makes it to ensure that it's doing all the things that it should be doing.

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‫So I'm going to click on this one.

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‫Let's look at a couple of things about Official Repositories.

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‫Like I said before, you're often going to be using officials to start with and then eventually you may

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‫find that you want to change it slightly or add a few things to it and you'll make your own images.

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‫Official images are a great way to start.

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‫One of the best things about official images is their documentation.

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‫They're always really great at documenting how to make them work,

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‫what options there might be, what environment variables, what's the default port and so on.

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‫So back up at the top, you'll notice these versions. Versions are a common feature of official repositories.

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‫You don't have to have versions in every image but official ones do.

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‫Because most open-source software there's always at least a few official versions out in the wild that

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‫are officially maintained and supported.

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‫That's what we have here. We have the mainline 1.11 for Nginx and then

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‫we have what's considered the stable branch of Nginx, which is 1.10. We can see the little name

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‫here says stable jessie Dockerfile.

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‫All right.

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‫So let's break this down.

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‫1.11.9, mainline, 1, 1.11 and latest.

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‫Those are all tags.

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‫When we start talking about images, images aren't necessarily named. Images are tagged. A version

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‫of an image can actually have more than one tag.

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‫We're going to dive into this a little bit later when we start making our own images and we can

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‫play around with tagging.

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‫But for now, if I wanted to actually download that specific image, and you remember earlier that we actually

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‫already downloaded it,

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‫so if I do a docker image ls, you'll see the three images that we've worked with so far on my machine.

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‫You see the tag that says latest. Now latest is a special tag.

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‫It doesn't necessarily guarantee it's always the latest commit in the repository.

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‫What it usually means is you're getting the latest version of this product. In the official

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‫repositories,

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‫it's very well-defined and consistent so that if you didn't really care right now exactly what version

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‫you wanted, you just wanted the most current, then you could just say docker pull nginx. It would

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‫download the latest. You'll notice that it already knew I had it, it checked the SHA to make sure it

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‫matched the one from Docker Hub and said "Hey I've already got this image,

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‫it's up to date." What if I wanted to download docker image version 1.11.9?

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‫It's going to go check and see what it needs to do to download that image

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‫and it turns out it doesn't need to do anything because it's the exact same version.

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‫That's what this line means up here.

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‫I can refer to this 'main, latest' version right here by any of these tags.

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‫The reason is if I just wanted to stay on the latest version of 1.0, whatever that might be for

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‫Nginx, I could just specify a 1. If I wanted to be more specific when I downloaded images and I wanted

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‫the most latest version of 1.11, then I could just type 1.11 like this and just

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‫leave out the .9 and it would download that one.

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‫But in my case, I actually specified the most specific of all the 1.11.9 which shows

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‫the point release and that was just to ensure that I got the exact version I wanted.

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‫So in your software, a best practice is when you're going to production and you're actually testing software

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‫that you're going to be using for others,

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‫you always want to specify the exact version.

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‫It's rare that you really want your software to update automatically.

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‫You usually want to control that process with some other DevOps tool.

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‫But when you're developing or just testing something locally, it's super easy with official images to

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‫just type in the name and just assume you're going to get the latest.

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‫So you'll notice here there's other ones.

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‫There's 1.11.9-alpine and these all have very similar names to the first one

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‫but they all have the word alpine in them.

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‫We're going to get into base images and distributions later, but just for now, Alpine is actually

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‫a distribution of Linux that's very very small.

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‫It's actually less than 5MB in size I believe. This version will actually mean that it comes from

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‫a base image of Alpine, keeping it very small and light, whereas the default one or the latest image actually

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‫comes from jessie, which is a Debian distribution. It's a little larger in size probably a little

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‫bit over 100MB.

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‫In fact, we can actually go over here and we can look.

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‫So nginx 182MB right here.

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‫If I want to download that exact version of the Alpine, I can say 1.11.9-apline

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‫and then do docker image ls and you'll see that the alpine version is actually only 54MB

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‫and the latest version is 182MB.

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‫You'll also notice that the three versions that I've downloaded all have the same image ID because

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‫the image ID is based upon the cryptographic SHA of each image in Docker Hub.

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‫So this isn't actually three different copies of 128MB images.

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‫It's actually a little deceiving.

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‫It's really just three tags of the exact same image ID and it's only total ticking up 182MB

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‫and these are just two other lines showing the same image ID with different tags associated to it.

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‫But enough about tags and versions for now.

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‫Let's check a few more things out

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‫in Docker Hub. If I go back up to nginx in the search bar and search for nginx again, you'll see

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‫again that there's a lot of other options and these can be made by anyone.

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‫You can create your own custom Nginx version and because it's open source software and free to

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‫distribute, you can put it back up here and offer it up to the public.

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‫This is very similar to the model of GitHub where they allow you to store open source software and

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‫you can fork other people's software and also offer it up.

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‫The same thing goes here.

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‫You can use other people's images and download them, make them your own, re-upload them.

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‫So if I ever want to consider not using an official repository, what I usually look for is a number

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‫of stars and a number of polls because a popular repository, to me, tends to establish trust.

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‫I always recommend you download and inspect the software before you use it. Look in their Dockerfile

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‫and hopefully they'll have an open source repository that you can go look at exactly how they

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‫made that image.

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‫A good indication is when you see things that have five million pulls on them and 945 stars; clearly

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‫there's something interesting about this package and why so many people are choosing it rather than

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‫using the official image.

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‫You can come in here and look at their repository.

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‫You can actually see over on the right that he is linked to his GitHub repository where the actual

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‫open source software is that builds this image so that you can go and inspect it; and, possibly download

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‫the actual source code outside of an image.

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‫But, he clearly has done something with this Nginx image that makes it better than the official

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‫default.

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‫A couple more things. On the top bar, you'll see Explore. Explore

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‫just gives you a summary of all the official images and you can peruse through them and, trust

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‫me, there's a lot. In fact, in the resources for this section,

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‫you'll find a link to the actual list of official images and under 'library,'

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‫this is where all the official images and their Dockerfiles live.

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‫You can see there's a lot. As a quick recap, in this lecture we talked about Docker Hub and how it's

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‫similar to being the apt package manager for Containers,

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‫Then how to recognize official images and how to use them.

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‫Then I helped you figure out how to determine a good public image from maybe a not-so-good one.

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‫Then a few tips about base images and how Alpine ones are typically smaller than their Debian counterparts.

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‫So we're going to use these skills in the rest of this section.