Building a Hello World Container Image

Objective

Create a Dockerfile that builds into a container image that outputs Hello World.
Run the container and verify it works.

Documentation: https://docs.docker.com/engine/reference/builder/
docker build . will build the Dockerfile in the current directory
- The . indicates that the Dockerfile's build context is this directory, and can not access any parent directories.
- The -f NAME parameter can be used to define what file to build. By default, a file called Dockerfile is being looked for.
- Use -t NAME to tag the built image with a name. This way that name can be used to later on run the container image.

Short template Dockerfile:

FROM ubuntu

ENTRYPOINT ["/bin/echo", "Hello"]
CMD ["World"]

The FROM IMAGENAME:VERSION defines what to use as the base image for the container image. Any changes on top of that will be additional changes to the base image.
The ENTRYPOINT defines what executable (and parameters) the container will launch with when ran.
The CMD defines the default command to run. The CMD parameter contents will be passed/suffixed to the entrypoint, so the example here becomes /bin/echo Hello World.
- It is possible to easily override the default command when running containers. For example, if this container were to be built and ran, we could override the CMD by doing docker run <image> Aslan. This would then replace the CMD with Aslan instead of World, and the complete command would result in /bin/echo Hello Aslan.