The auto-injected GITHUB_TOKEN isn't authorized against this Gitea
instance's container registry (401 unauthorized), so use a repo
secret backed by a personal access token with package scope instead.
Gitea rejects variable/secret names starting with GITEA_ or GITHUB_
(reserved for system-injected values), which is why GITEA_REGISTRY
couldn't be created in repo settings.
act_runner's default registration only exposes ubuntu-latest (and
similar ubuntu-* labels); "docker" isn't a real label unless a runner
was explicitly configured with it.
jeanGaston