Migrate from CircleCI and Docker Hub to Github Actions and GHCR

[s3lph]

• build job successfully tested here: https://github.com/SpaceApi/directory-api/actions/runs/13093142620/job/36532148709
• publish (then named build) job successfully tested here: https://github.com/SpaceApi/directory-api/actions/runs/13092989253/job/36531846510
• Resulting images:
  • ghcr.io/spaceapi/directory-api
  • ghcr.io/spaceapi/directory-collector
• The pipeline status of all commits in this branch shows as failed, because the CircleCI job that's still enabled (but its config removed) fails as expected.
  • CircleCI should be disabled for this repo, and the webhook removed directly before this is merged.
• Should be squashed and merged.