Builds a tiny inline alpine image through build-and-push against the
remote buildx builder and asserts the pushed manifest is multi-arch.
Pushes to a throwaway :citest tag so it never touches real images.
Triggered on workflow_dispatch and on changes to the action.
buildx refuses a builder instance whose name matches an existing docker
context. Create the builder directly from the ssh:// endpoint and clean up
any leftover context/builder of the same name first (idempotent re-runs).
- DOCKER_HOST env var was set to the builder hostname, which Docker treats
as tcp://host:2375 and which overrides the active docker context, so the
SSH remote-builder context was ignored (Cannot connect to tcp://docker-build.haven:2375)
- context endpoint used a literal ssh://***@ instead of the docker_user input
- create an actual buildx builder instance named remote-builder (build-push-action
needs a builder instance, not just a context)
