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‫Let's dive into the Dockerfile, which is actually a recipe for creating your image.

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‫So far, all of the Docker images we've been using were created from Dockerfiles where you can go look

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‫at them on Docker Hub.

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‫If you've cloned my repository from the resources listed in the first section you should see a

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‫docker sample file 1 directory, and in there we should have just one file. A Dockerfile.

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‫We're going to go line by line and talk about these different stanzas. A Dockerfile seems to look

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‫like a shell script, but it's not.

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‫It's not a batch file, it's not a shell script. It's a totally different language of file that's unique

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‫to Docker and the default name is Dockerfile with a capital D.

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‫But, from the command line, whenever you need to deal with a Dockerfile using the docker command, you can

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‫actually use the dash f. Which is actually common amongst a lot of the tools with Docker, you can use

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‫dash f to specify a different file than a default.

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‫So, first up is the from command. The from command is in every Dockerfile. It's required to be there.

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‫It's normally a minimal distribution.

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‫In this case it's Debian.

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‫But a lot of times nowadays it's going to be Alpine.

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‫And really, the reason you would use these is to save yourself time and pain. Because these minimal distributions

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‫are actually much smaller than the CDs you would use to install a virtual machine from them.

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‫For example, the Ubuntu one doesn't even have cURL in it, whereas obviously, if you installed

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‫a full Ubuntu installation on a VM it would have cURL and a lot of other commands already installed.

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‫Because all of these distributions are official images, it means that they're going to be always up

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‫to date with the latest security patches and you can depend and trust on them.

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‫And one of the main benefits for using them in containers is to use their package distribution systems

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‫to install whatever software you need in your package.

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‫In this case, we're actually using the Jessie release, which is what's after the colon.

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‫And, as you remember from our Docker Hub conversation, there's lots of different options you can specify

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‫here for the Debian repository.

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‫Next, we have the ENV stanza, and ENV is for environment variables.

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‫It's a way to set environment variables, which are actually very important in containers because they're

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‫the main way we set keys and values for container building and for running containers.

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‫In this case, it's actually setting the version of Nginx it would like us to install and this environment

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‫variable will be set so that any subsequent lines will be able to use it.

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‫Now, as a reminder from previous lectures, each one of these stanzas is an actual layer in our Docker

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‫image.

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‫So the order of them actually matters, because it does work top down.

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‫As you'll see by this relatively long run command here for the next one. You’ll see that it looks like

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‫one long command.

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‫What it is doing is really just executing shell commands inside this container as it's building

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‫it.

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‫You'll usually see run commands when you need to install software with a package repository, or you

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‫need to do some unzipping, or some file edits inside the container itself.

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‫Run commands can also run shell scripts that you've copied in earlier in the file or any commands that

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‫you can access from inside the container

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‫at that point in time in the file.

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‫Since we're coming from Debian, this run command has access to all the commands and binaries that

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‫would have been installed with that release.

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‫And in this one for Nginx, this list of commands is more or less adding a key to the repository where

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‫you can get the package to install the latest Nginx from the official Nginx repository.

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‫There's a few things here that are really key in making good Dockerfiles and we'll actually talk

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‫about this more in a later section where we talk about best practices.

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‫But two things to note.

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‫The reason that we're adding all these commands with the double ampersand here, so that they're chained

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‫one after the other, is because, if you remember, each stanza is its own layer.

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‫What this does is ensures that all of these commands are fit into one single layer.

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‫It saves us a little time.

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‫It saves us space. And it's so common that you'll probably see it in every Dockerfile on Docker Hub.

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‫Okay, on line 27 we've got another run command.

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‫This one is all about pointing our log files to the stdout

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‫and to the stderr.

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‫We'll see later, the proper way to do logging inside a container is to not log to a log file. And there's

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‫no syslogd or any other syslog service

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‫inside a container. Docker actually handles all of our logging for us. All we have to do inside the

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‫container is make sure that everything we want to be captured in the logs is spit to stdout and stderr

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‫and docker will handle the rest.

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‫There's actually logging drivers that we can use in the Docker Engine itself to control all the logs

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‫for all the containers on our host.

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‫And so that's really what you want to do with them.

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‫It adds more complexity to your app if your app is actually doing the logging itself.

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‫And then, if you have to deal with files in every container, now you've got a problem of how do you get those

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‫files out, and searchable, and accessible. Here, we're taking the default Nginx logs and we're actually

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‫linking them to the stdout.

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‫So that Docker can capture them.

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‫We also have an expose command.

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‫By default, no TCP or UDP ports are open

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‫inside a container. It doesn't expose anything from the container to a virtual network unless we list

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‫it here.

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‫And, of course, because this is a Web and proxy server it's going to expose 80 and 443.

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‫Now, this expose command does not mean these ports are going to be opened automatically on our host.

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‫That's what the dash P command is whenever we use docker run.

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‫Lastly, we have the CMD here. The CMD is a required parameter that is the final command that will

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‫be run every time you launch a new container from the image, or every time you restart a stopped container.

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‫There is some really excellent documentation on all of these stanzas, plus a whole lot more, that

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‫we're going to go into later, on the Docker documentation website at docs.docker.com,

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‫which you can find in the resources of this lecture.

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‫Okay, so these are the basics, right? These five different stanzas are pretty normal in every single Dockerfile.

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‫Some of them are required.

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‫Some of them, like the run, and the ENV, and the expose, are optional, but they're pretty typical for most

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‫containers that you're going to create images for.