CI: Pre-pull docker images to optimize caching

Also, switch to pierrezemb/gostatic:latest instead
of specific hash due to
satackey/action-docker-layer-caching#17

.github/workflows/ci.yml

@@ -29,10 +29,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1
+    - run: >
+        echo ::set-env name=K8S_EMUL_IMAGE::kyryloshpytsyaprismai/k8s-emul:sha-${GITHUB_SHA::7}
+    - name: Pull Docker images
+      run: >
+        cd tests &&
+        docker-compose pull
     - name: Pull Docker layer cache
       uses: satackey/[email protected]
     - name: Run tests
       run: >
         sudo
-        K8S_EMUL_IMAGE=kyryloshpytsyaprismai/k8s-emul:sha-${GITHUB_SHA::7}
+        --preserve-env=K8S_EMUL_IMAGE
         ./run-tests

tests/test-in-docker

@@ -43,7 +43,7 @@ cat >test-job.json <<'_END_'
             "Driver": "docker",
             "User": "",
             "Config": {
-              "image": "pierrezemb/gostatic@sha256:e28d48e17840c5104b5133c30851ac45903b1d2f268d108c4cd0884802c9c87e",
+              "image": "pierrezemb/gostatic:latest",
               "args": [
                 "-enable-health"
               ],