Use uv sync --frozen so Docker builds consume the pinned uv.lock instead
of resolving issue-core from Forgejo PyPI before that package is mirrored.
Align image.yaml with the canonical container-build template.
Add image.yaml workflow targeting forgejo.coulomb.social; update Dockerfile
for Forgejo PyPI secret names. Point pyproject issue-core index at Forgejo
(uv.lock regen blocked until issue-core is published there).
