Reenable dockerhub builds

This MR supersedes !1142 (closed) since we will move RPM packaging into a branch off of release/5.0. However dockerhub builds should be performed on master and release branches, so this MR extracts those aspects into this branch.

For dockerhub the strategy that is adopted:

• builds will occur on master, any branch tagged docker/* for testing purposes, and any tags (to handle patch releases).
• images are tested in a package-test phase, using a simple IncNavierStokesSolver test with MPI, and by using the executable template, which also tests that Nektar++ can be used as a third-party library.
• if all of that passes then the images are pushed up to dockerhub, asides from if you are on a docker/* branch for testing.
• The gitlab container registry is used to store intermediary images and deployment only occurs if tests pass.

To enable RPM packaging, this MR also introduces a couple of changes to enable installation of headers in a separate environment.