ci: add GitHub Actions to test current branch

Author: mcepl

.github/workflows/build.yml

@@ -35,7 +35,7 @@ jobs:
     if: needs.check_source.outputs.run_tests == 'true'
     # Use a specific image version for reproducible builds
     container:
-      image: opensuse/leap:15.5
+      image: opensuse/leap:15.6
     strategy:
       fail-fast: false
       matrix: