GitHub Actions stopped being reachable when the account was suspended, so
this ports ci.yml and publish.yml to a .gitlab-ci.yml running on a group
runner on Web_Host. The Actions workflows stay in the tree: they are the
reference this was written from, and they work again unchanged if the
appeal succeeds.
Two differences worth knowing. Images are pinned by digest rather than the
workflows' SHA-pinned actions, because GitLab has no action allowlist to
back a tag with. And arm64 is built under QEMU instead of on a native
runner, which is slow enough that publish is tag-only.
