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Dockerfiles

Docker isn't only useful for running other people's software (as we've been doing so far). It's also a great way to build and package our own software.

I've used Docker both ways. As a DevOps/platform engineer I'm usually using other's images, but as a backend developer I was usually building images for our own servers.

Docker images are built from Dockerfiles. A Dockerfile is just a text file that contains all the commands needed to assemble an image. It's essentially the "Infrastructure as Code" (IaC) for an image. It runs commands from top to bottom, kind of like a shell script.

Instead of manually installing dependencies on servers and making updates manually, we can check a Dockerfile into source control and build it automatically. Mhmmmm, automation.

Assignment

Create a file called Dockerfile in your working directory. If you're using VS Code, consider installing the Docker extension for syntax highlighting.
Inside the Dockerfile add these lines of text:

# This is a comment

# Use a lightweight debian os
# as the base image
FROM debian:stable-slim

# execute the 'echo "hello world"'
# command when the container runs
CMD ["echo", "hello world"]

Build a new image from the Dockerfile and call it helloworld:

docker build . -t helloworld:latest

The -t helloworld:latest flag tags the image with the name "helloworld" and the "latest" tag. Names are used to organize your images, and tags are used to keep track of different versions.

Run your image in a new container:

docker run helloworld

If all went well, you'll see "hello world" printed to the console!

Run docker ps. You'll notice that your container is not running anymore! All it did was print and exit. Just like regular programs, docker containers can execute simple commands that exit quickly, or they can execute servers that run until killed. It just depends on the command you give it.
See the stopped container with docker ps -a.
Delete the Dockerfile, we don't need it anymore.

Run and submit the CLI tests.