feat: /version endpoint; fix deploy action inputs

- Bake the commit SHA into the image (GIT_SHA build arg) and serve it at
  /version, so a deploy can be verified end to end. This repo is the
  dockerloid deploy test service.
- cssnr/portainer-stack-deploy-action@v1 reads `endpoint` and `pull`;
  `endpoint_id` / `pull_image` were silently ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -10,6 +10,10 @@ COPY app.py ascii_engine.py ./
 COPY templates ./templates
 COPY static ./static
 
+# Build provenance, served at /version so deploys can be verified
+ARG GIT_SHA=unknown
+ENV GIT_SHA=$GIT_SHA
+
 # Expose port
 EXPOSE 5000