Docker recipe update procedure + README instruction.

[dreamer2368]

Docker container dreamer2368/librom_env is now moved into the current repository's container registry. The new docker container is located at ghcr.io/llnl/librom/librom_env.

• Minor update within the container:
  • mpi4py python package is pre-installed.
  • lldb, gdb added.
• Instruction for using the docker container.
• The new CI test .github/workflows/docker.yml detects any change in docker/Dockerfile or the test file itself, and automatically update/push the docker container ghcr.io/llnl/librom/librom_env.
  • This will be invoked only when there is a change.

[chldkdtn]

@michael-barrow-llnl and @siuwuncheung let's review this.