Changes to Docker build to improve dev workflows

[GhostofGoes]

Summary

The primary reason for this PR is to allow usage of non-GitHub repos to be used for APPS_REPO during the Docker image build. There are also a number of other enhancements to the Dockerfile and a handful of updates to the README (just some cleanup while I was in the area).

Changes

• APPS_REPO must now include the base domain for the repo. By default, this is github.com. The change should have minimal impact on existing workflows. This change enables use of other Git hosts for phenix-apps.
• Add some labels to the image metadata. This is a best practice that we use for several other projects.
• Disable pip update checks
• Disable pip caching
• Move scorch dependency installs before python installs, to improve caching (since these usually don't change, so docker buildkit can cache and not have to pull when rebuilding)
• Add ability to configure pip index via build arg. This enables use of PyPI mirrors to speed up builds and dodge proxy errors.
• Remove --trusted-host arguments from pip. There's no reason to have these if the certificates are installed.
• Add a few comments explaining stages
• Update APPS_REPO in the docker GitHub Action workflow
• Add .venv to .gitignore
• Minor changes to README.md

[GhostofGoes]

@activeshadow Pushed changes to address review comments. These changes have been tested.