fix: Dockerfile build mem-cli binary + CI builds image on PRs

- rust:1.81 → rust:1-bookworm (track latest stable)
- cargo build --release → cargo build --release -p mem-cli
- Docker build runs on PRs (verify before merge)
- Push only on main/workflow_dispatch

.gitea/workflows/build.yaml

@@ -43,6 +43,7 @@ jobs:
             -f Dockerfile .
 
       - name: Registry login
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
         run: |
           echo "${REGISTRY_TOKEN}" | docker login "${REGISTRY}" \
             --username "${REGISTRY_USER}" --password-stdin
@@ -51,10 +52,12 @@ jobs:
           REGISTRY_TOKEN: ${{ secrets.FORGEJO_REGISTRY_TOKEN }}
 
       - name: Push Docker image
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
         run: |
           docker push "${IMAGE}:${{ steps.sha.outputs.short_sha }}"
           docker push "${IMAGE}:latest"
           echo "✓ Pushed: ${IMAGE}:${{ steps.sha.outputs.short_sha }}"
 
       - name: Prune unused images
+        if: github.event_name == 'push' || github.event_name == 'workflow_dispatch'
         run: docker image prune -a --force 2>&1 | tail -3 || true