build(docker): Refine the .dockerignore as a whitelist

[erikmd]

• Kind: cleanup

Description

• many potential folders/files to ignore (e.g. _build) were not listed
• let's bluntly ignore what is not required by the Dockerfiles

Checklist

• Read the CONTRIBUTING.md guide and:
  • Use Atomic Commits so each commit gathers a single logical change
  • Use Conventional Commits regarding commit messages (needed by our release toolchain)

Note to maintainers

• Read this wiki page
• Make sure the PR has a milestone
• Assign yourself before merging
• We can squash-merge 1-commit PRs (use a header with a conventional-commit type, add a footer with Fix #… if need be)

[erikmd]

All CI workflows were fine, so merging this small cleanup PR.