The privileged runner's job image has no Docker client, which the login
and build actions need. The registry host can be set with a REGISTRY_HOST
repository variable when the runner reaches Gitea at an internal address.
A Gitea workflow builds the app, session manager and streamer images
and pushes them to the instance's registry on pushes to main and on
version tags, reading the registry host at run time. Compose can pull
them with IMAGE_PREFIX. The update checker reads APP_UPDATE_URL instead
of upstream's GitHub releases and is off without it. Upstream's Docker
Hub workflow is removed.
branderson