Migrate from CircleCI and Docker Hub to Github Actions and GHCR

[s3lph]

• test job successfully tested here: https://github.com/SpaceApi/validator/actions/runs/13094621565
• publish job successfully tested here: https://github.com/SpaceApi/validator/actions/runs/13094621566
• Resulting image: ghcr.io/spaceapi/validator
• 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.

[dbrgn]

LGTM!

The name of the docker image could be extracted into a variable for easier sharing of configs. But not required for merging.