Improve documentation for building/testing the release candidate container

The README/documentation for building the release candidate container requires cloning the daffodil repository and making sure it is fully up to date. That's not difficult, but it a potential source of non-reproducible builds if someone forgets to update the repo or if there are locally made changes.

The podman build commands accepts a path to a git repo, and it will automaticaly clone the specified branch and build from that. This ensures we build with the latest container and without local changes.

We should also include documentation in the README for how to and build and test local changes without publishing anything.