Throwaway. Deleted once #237 has an answer, whichever way it goes.
#233 was designed on an assumption about the Docker build context and broke every Portainer deploy. The chosen fix for the commit stamp — build in Actions, push to Gitea's registry, have Portainer pull — rests on three more assumptions about infrastructure that nothing in this repository can confirm. This probes them instead of designing around them.
Whether a job here can run `docker build` at all is genuinely unknown: the other workflows use `services:`, which proves the runner can start containers, not that it can build images. Whether the registry is usable is likewise unconfirmed — the packages API answers but lists nothing, so it has never been exercised. And the real build is timed because backend-integration.yml is manual after a job once held this runner for 3h12m, so what an image build costs here is part of deciding whether building on every merge is tenable at all.
The trivial image is built and pushed before the real one on purpose. It separates "can this runner build and push anything" from "does our Dockerfile work here", so a later failure still says which half is broken.
workflow_dispatch only, with no push or pull_request trigger, so merging this changes nothing until somebody presses the button. Bounded by timeout-minutes on the same reasoning backend-integration.yml already documents.
Ref #237
