ci: changed gh actions to avoid image push on PR

.github/workflows/pr.yml

@@ -0,0 +1,19 @@
+name: PR validation
+
+on:
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Build Docker image
+        uses: docker/build-push-action@v1.1.0
+        with:
+          tag_with_sha: true
+          cache_froms: itzg/minecraft-server:latest
+          push: false