Install gala from source with EXP support

[hfoote]

Updates the Dockerfile with the following changes:

• gala is now installed from source, instead of from PyPI. This allows building gala with EXP support according to the instructions at https://gala.adrian.pw/en/latest/tutorials/exp.html

• Removed a checkout of the haloVel branch of pyEXP-examples, as this branch no longer exists.

NOTE: This update requires that the bug fix in PR 218 in the EXP repo is implemented, so that gala compatibility with the main branch of EXP is restored.

[The9Cat]

Looks good to me.

To Do list

• Successful CI build and merge to main of missing public headers PR
• Run the Docker build action to check Docker build
• Update HPCCM control file for consistency with these changes and confirm that the resulting Dockerfile is correct and builds
• Check Gala has the expected EXP support

[The9Cat]

Looks good to go. Easiest to merge and then check the runner.