Fix interference between build_doxygen and package_image jobs

!250 (merged) moved the Doxygen generation stage to a new job in the build stage. The artifacts (build/doc/html) from this were then automatically copied to the Docker build job (package_image) in the package stage. The presence of the build/ directory confused the build inside Docker.

The fix is to remove the dependencies on the package_image job such that it does not get any artifacts. This was done with an empty needs keyword. A nice side effect is that this job will also start before the full build stage completes.

This wasn't caught during the !250 (merged) CI as the package_image job only runs in the main branch. See pipeline for this MR passing with all jobs enabled.