build: Switched to GHA cache (#1153)

.github/workflows/pr.yml

@@ -4,6 +4,9 @@ on:
   pull_request:
     branches: [ master ]
 
+env:
+  IMAGE_TO_TEST: itzg/minecraft-server:test-${{ github.repository_owner }}-${{ github.run_id }}
+
 jobs:
   test:
     runs-on: ubuntu-latest
@@ -11,6 +14,19 @@ jobs:
     steps:
       - uses: actions/checkout@v2.4.0
 
+      - name: Setup Docker Buildx
+        uses: docker/setup-buildx-action@v1
+
+      - name: Build
+        uses: docker/build-push-action@v2.7.0
+        with:
+          context: .
+          platforms: linux/amd64
+          tags: ${{ env.IMAGE_TO_TEST }}
+          load: true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
       - name: Run tests
         run: |
           tests/test.sh