On push to main send a repository_dispatch event to chatmail/docker with
relay_ref, relay_sha, and relay_sha_short.
This triggers docker-ci.yaml to build a new Docker image from
the updated relay code, push to GHCR, and eventually run integration
tests via cmlxc's reusable lxc-test workflow.
Requires DOCKER_DISPATCH_TOKEN secret with repo scope on
chatmail/docker.
Also set workflow_dispatch to allow manual triggering of Docker builds
from any relay branch via the GitHub UI.
