github actions: test container build with podman on PRs

Signed-off-by: Michael Adam <obnox@redhat.com>

.github/workflows/main.yml

@@ -29,6 +29,18 @@ jobs:
     - name: Run checks
       run: make check
 
+  podmanbuild:
+    runs-on: ubuntu-latest
+    # don't run on push, since the "push" job contains the
+    # image build step, so no need to do it twice.
+    if: github.event_name == 'pull_request'
+    steps:
+      - uses: actions/checkout@v2
+      - name: build container image
+        # note: forcing use of podman here since we are
+        # using podman explicity for the push job
+        run: make CONTAINER_CMD=podman image-build
+
   # push the container to quay.io - only for pushes, not PRs
   push:
     needs: [build, check]