Creating an Image Including Files

The image we just created in the previous lesson didn’t need anything other than what the base image already contained, which is why our Dockerfile file was so simple. In a real-world scenario, however, I’m very likely to want files to be part of an image I create.

Suppose I have a file named index.html on my disk with the following contents:

index.html

I want to create an image that includes a web server that serves the above page over HTTP. NGINX is a good candidate. I could keep using the debian base image and add instructions to my Dockerfile file that install NGINX, but it’s easier to base my work on images that are already configured and tested. The Docker Hub contains an NGINX image where NGINX has already been installed with a configuration that serves files found in the /usr/share/nginx/html directory.

I create the following Dockerfile file in the same folder as the HTML file:

Dockerfile

Apart from the nginx base image, you can see a COPY instruction. Its first parameter is the file to be copied from the build context and its second parameter is the destination directory inside the image.

The build context is basically the directory you provide to the docker build command. Its contents are available for COPY instructions to use, but only during the image build process. That means it’s available only for the instructions in the Dockerfile file, and files from it won’t be a part of the build image or containers that you’ll spawn from that image unless you use the COPY instruction. This is why we have a COPY instruction; we want the index.html file to be part of the /usr/share/nginx/html directory inside the image we create.

You may have noticed that this time the Dockerfile file contains no CMD instruction. Remember that the CMD instruction states which executable should be run when a container is created from my image, so it’s weird that I don’t include a CMD instruction that runs an NGINX server. The reason why I didn’t include a CMD instruction is because the base nginx:1.15 image already contains a CMD instruction to run the NGINX server. This is part of my image and I don’t need to include my own CMD instruction as long as I don’t want to run another executable on container startup.

Back to creating our HTTP server image, I open a command line inside the folder where my index.html and Dockerfile files are, and run the following commands:

The above commands build a webserver from the Dockerfile file instructions, then start a container that listens to my machine’s 8082 port and redirect the incoming connections to the container’s 80 port. You can start a browser and point it to http://localhost:8082 to view it locally. This displays the HTML file contents in the browser since they are served over HTTP by the running container:

In the terminal tab, you can see a log from NGINX that proves it received the HTTP request from my browser:

172.17.0.1 - - [.../2019:21:14:46 +0000] "GET / HTTP/1.1" 304 0 "-" "..." "-"

When running my container, I added the –rm and -it switches simply for demo purposes. In reality, that server container would be long-running, so I’d run it without those switches. Here’s why I used the switches:

• The -it switch allows me to stop the container using Ctrl-C from the command-line

• The –rm switch ensures that the container is deleted once it has stopped

Let’s continue our discussion on image creation in the next lesson.