CI: refactor Docker build jobs

Context

This refactoring simply reorders the fields / extends / variables in all the build/docker_*.yml files so that they are in the same order. This makes it a lot easier to understand the difference between them through visual inspection. I've checked with GitLab CI Inspector that no semantic change was re-introduced.

After:

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR