ci: cache docker layers and avoid re-downloading them for each job

Sometimes the CI fails because Docker doesn't respond.
It could be because we download many docker images in every
job and we have 58 jobs now.
With this commit, the images layers will be downloaded
during nightly builds and cached.
The CI will still run against the latest containers available because
we don't apply tags to layers. So if something changed in Docker Hub
we will always use the new layers in place of or (hopefully) on top
of the cached layers.

This seems to cut the CI running time in half from 33~ minutes to 15~.

Signed-off-by: Salvatore Mesoraca <[email protected]>

.github/workflows/integration.yaml

@@ -50,8 +50,28 @@ jobs:
            pip install -r dev_requirements.txt
            invoke linters
 
+   populate-cache:
+     runs-on: ubuntu-latest
+     timeout-minutes: 60
+     name: Update docker cache
+     steps:
+       - uses: actions/checkout@v4
+       - name: Cache docker images
+         id: custom-cache
+         uses: actions/cache@v3
+         with:
+           path: ./custom-cache/
+           key: custom-cache
+       - if: ${{ steps.custom-cache.outputs.cache-hit != 'true' || github.event_name == 'schedule' }}
+         name: Update Cache
+         run: |
+           mkdir -p ./custom-cache/
+           docker compose --profile all build
+           docker save $(docker images -q) -o ./custom-cache/all.tar
+
    run-tests:
      runs-on: ubuntu-latest
+     needs: [populate-cache]
      timeout-minutes: 60
      strategy:
        max-parallel: 15
@@ -70,6 +90,17 @@ jobs:
          with:
            python-version: ${{ matrix.python-version }}
            cache: 'pip'
+
+       - name: Cache docker images
+         id: custom-cache
+         uses: actions/cache@v3
+         with:
+           path: ./custom-cache/
+           fail-on-cache-miss: true
+           key: custom-cache
+       - name: Use Cache
+         run: docker image load -i ./custom-cache/all.tar
+
        - name: run tests
          run: |
            pip install -U setuptools wheel
@@ -115,6 +146,17 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: 3.9
+
+      - name: Cache docker images
+        id: custom-cache
+        uses: actions/cache@v3
+        with:
+          path: ./custom-cache/
+          fail-on-cache-miss: true
+          key: custom-cache
+      - name: Use Cache
+        run: docker image load -i ./custom-cache/all.tar
+
       - name: Run installed unit tests
         run: |
           bash .github/workflows/install_and_test.sh ${{ matrix.extension }}