Cache Docker image used in build

Author: mrotteveel

.github/workflows/run-tests.yml

@@ -10,6 +10,11 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
+      - name: Cache Docker images
+        uses: ScribeMD/docker-cache@fb28c93772363301b8d0a6072ce850224b73f74e
+        with:
+          key: docker-${{ runner.os }}-${{ hashFiles('.github/workflows/**') }}
+          read-only: ${{ github.ref != 'refs/heads/master' }}
       - uses: juarezr/[email protected]
         with:
           version: '5.0.2'