fix(ci): simplify Push step to match auth pattern (CAR-1362)

The Push Docker image step is failing post-merge of CAR-1362 with
buildx "unknown" error after layers push successfully. The pre-existing
failure was masked by the cache export error.

Simplify the Push step to match the proven-green cartsnitch/auth/ci.yml
pattern: drop `file: ./Dockerfile` (default) and `build-args:`
(APT_CACHE_BUST is only used to bust apt cache in stage 1 of multi-
stage build, not needed for the rebuilt image). Keep `if: github.event_name
== "push"` to skip on pull_request events.

Diff: 4 lines removed from .gitea/workflows/ci.yml Push step.

Co-authored-by: Paperclip <noreply@paperclip.ing>

.gitea/workflows/ci.yml

@@ -160,12 +160,9 @@ jobs:
         uses: docker/build-push-action@v6
         with:
           context: .
-          file: ./Dockerfile
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
-          build-args: |
-            APT_CACHE_BUST=${{ github.run_id }}
 
       - name: Create git tag
         if: github.event_name == 'push' && github.ref == 'refs/heads/main'