Investigate whether stages pipeline variable can be removed

AzDO now supports the selection of which stages should be run when queuing a build. In theory, this would mean the [`stages` variable](https://github.com/dotnet/docker-tools/blob/f5dc3c6f4bc8ad430fe4faae000dfd77fdd04927/eng/common/templates/stages/build-test-publish-repo.yml#L23) could be obsolete. This should be investigated to see if we can now get rid of this variable.