handle CLAS12 software versioning

• for now, the build scripts use whatever the latest main branch is, i.e., the true bleeding edge; this is not a great idea, especially for repos that do not keep the main branch stable, so we should instead use fixed versions
• agreed with @baltzell to source version numbers from clas12-env
  • the clas12 module files could be modified to read the version numbers from some common file, so that both clas12-env and clas12-containers can just read that
• 1st argument of build scripts should be the version number
• version number is preferably a git tag, but may also be a commit hash
• we want to publish the version numbers for any tagged release of clas12-container since those images won't be removed from the registry (preservation)
  • furthermore, the tag that we use here should be in sync with the clas12 modulefile version