ci: name the Containerfile in the build smoke step

Run #2 got through Checkout, the runtime probe and the whole suite (157 passed,
3 skipped — the `node --check` structural tests skip with no node in a python
image, which is the graceful degradation they were written to have) and then
failed on `docker build -t mirror:ci .`: with no -f, docker looks for
"Dockerfile", finds none, and says "failed to read dockerfile" after
transferring a 2-byte context. The publish job never hit this because
docker/build-push-action passes `file: Containerfile` on its own.

The gate is worth keeping rather than dropping: it is the only step that builds
the image on a push, so without it a broken Containerfile surfaces only as a red
publish job that quietly leaves :main stale.

LLM-Generated: true

.gitea/workflows/build-and-push.yml

@@ -68,7 +68,11 @@ jobs:
         # install step) here rather than in the publish job below.
         run: |
           set -euo pipefail
-          "${{ steps.runtime.outputs.path }}" build -t mirror:ci .
+          # -f is not optional: without it docker looks for "Dockerfile", finds
+          # none, and reports "failed to read dockerfile" with an empty context —
+          # which is exactly how run #2 failed. The publish job gets away without
+          # naming it because docker/build-push-action passes `file:` for us.
+          "${{ steps.runtime.outputs.path }}" build -f Containerfile -t mirror:ci .
           "${{ steps.runtime.outputs.path }}" run --rm mirror:ci --help >/dev/null
 
   build-and-push-app: