Simplify the creation of Docker/Singularity images

[pzehner]

Given the recent security breach that affected the tj-actions/changed-files GitHub action, this was a good opportunity to reevaluate the way the project manages the creation of Docker images.

Currently, we have 2 sets of images: one for when the Dockerfiles are not modified (prefixed _main), and one for when the Dockerfiles are modified (prefixed _pr). Any feature branch that updated the Dockerfiles would then re-create its images on the fly. While very flexible, this process has not proven to be intensively used. Updating the Docker images is a somehow rare task. On the other hand, it generated a lot of complexity in the CI files.

This PR proposes to simplify the creation of Docker/Singularity images by separating the workflow between create_images, that only takes care of creating the images when a Dockerfile is modified, and the traditional build_test, that only uses the images. The registry will then only have one set of images, without suffixes.

If images need to be modified, then this should be done in a 2-step approach:

• Step 1: Create a PR that updates or creates the image; have it merged; and
• Step 2: Create a PR that uses the new image.

Progress so far:

• Modify the CI files;
• Force the creation of images without suffix;
• Finalize the CI files.