fix(multi-target-docker-images): split logic into multiple files

Closes #90 (closed)

Description

As mentioned in #90 (closed), the current implementation does not work. Somehow, it looks like if MULTI_DOCKER_IMAGE_BUILD_ENABLED is defined in variables, the value of it in rules is then taken from what was set in variables. The same goes for moving setting MULTI_DOCKER_IMAGE_BUILD_ENABLED in parallel:matrix.

This change goes back to the previous implementation where we split the logic into multiple files.

Working pipeline:

• https://gitlab.developers.cam.ac.uk/uis/devops/laboratory-allocator/lab-allocator/-/pipelines/636990