Chore: publish to dockerhub

[cgetzen]

This adds dockerhub as an image repository for ultar.

Additionally, it allows images to be created on non-release workflows. This should be available in the Actions UI post-merge, and was tested pre-merge via:

curl -X POST \
  -H "Accept: application/vnd.github+json" \
  -H "Authorization: Bearer $GITHUB_TOKEN" \
  https://api.github.com/repos/meshy-dev/ultar/actions/workflows/ci.yml/dispatches \
  -d '{"ref":"chore-push-to-dockerhub","inputs":{"docker_build":"true"}}'