Docker container workflows

[kathoef]

For example, Dockerfile vs. docker commit, this can have an impact on layer sizes!

Also, add description on how to interactively work with Docker containers, i.e. add missing software on the fly, but then put it into the Dockerfile and re-build after the work session.

There should also be a collection of ways to migrate a container image that was used locally to any remote machine, that has Docker and/or Singularity installed. Here, both the hub approaches, as well as local registry workflows should be mentioned. Also, how to convert a Docker image on a machine that doesn't have Singularity installed should be mentioned.