Simple Docker build on master pull request

.github/workflows/build.yml

@@ -0,0 +1,26 @@
+name: Docker build
+on:
+  pull_request:
+    branches: [master]
+    paths-ignore:
+      - .github/workflows/buildx-branch.yml
+      - .github/workflows/buildx-latest.yml
+      - .github/workflows/buildx-release.yml
+      - .github/workflows/dockerhub-description.yml
+      - .github/workflows/rust.yml
+      - extra
+      - _config.yml
+      - .gitignore
+      - .rustfmt.toml
+      - .gitignore
+      - example.json
+      - LICENSE
+      - README.md
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Build image
+        run: docker build .