Derive CI registry coordinates from workflow context
Drop the vars.REGISTRY gate and the REGISTRY/IMAGE_NAME repo-var
references that have been silently skipping the image job since the
spacebee rename. Registry host is derived from GITHUB_SERVER_URL and
image name from GITHUB_REPOSITORY, lowercased for docker safety. Only
REGISTRY_USER / REGISTRY_TOKEN secrets need to be set — forks on any
Forgejo instance work with no repo-var configuration.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>