github actions: podmanbuild: some free space debugging

Jobs are failing due to "no space left on device" when building
the container image. Adding some df output.

See: actions/runner-images#709

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

.github/workflows/main.yml

@@ -36,6 +36,8 @@ jobs:
     if: github.event_name == 'pull_request'
     steps:
       - uses: actions/checkout@v2
+      - name: check free space
+        run: df -h
       - name: build container image
         # note: forcing use of podman here since we are
         # using podman explicity for the push job