Avoid redundant builds

Author: AlistairB

.github/workflows/debian.yml

@@ -9,6 +9,11 @@ on:
       - '**/slim-buster/Dockerfile'
       - '.github/workflows/debian.yml'
 
+# See: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#concurrency.
+concurrency:
+  group: ${{ github.head_ref }}-${{ github.workflow }}
+  cancel-in-progress: true
+
 jobs:
   build-smoke-test:
     timeout-minutes: 30