Add GitHub workflow: Docker build.

Author: gendx

.github/workflows/docker-build.yml

@@ -0,0 +1,9 @@
+on: [push, pull_request]
+name: Docker build
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Docker build
+        run: ./docker-build.sh