Add Docker utilities

[Erik Schnetter]

This pull request https://bitbucket.org/cactuscode/utilities/pull-requests/3/add-docker-utilities adds instructions and auxiliary files for maintainers to create Docker images holding all Cactus dependencies.

There are also scripts to make it easy to use these Docker images. For example, the script ./utils/Docker/make-image-cpu/run-container-cpu launches Bash in a container using such a Docker image containing Ubuntu 20.04 with all Cactus dependencies installed, including AMReX. The Cactus source tree itself lives in the user’s home directory, only the dependencies live in the Docker image. The Docker image is not modified. The container is ephemeral, holds no state, and is destroyed when exiting the shell.

[Roland Haas]

There were added to CactusAMReX for a maturation period instead. In git hash fb6d70d "Add utilities to create and run Docker images with Cactus dependencies" of cactusamrex