Create docker-image.yml

Builds automatically `15 * 1,15 * *` (At minute 15 on day-of-month 1 and 15.).

.github/workflows/docker-image.yml

@@ -0,0 +1,21 @@
+name: Docker Image CI
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '15 * 1,15 * *' # Runs only on default branch
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Build the Docker image
+      run: docker build . --file Dockerfile --tag php-imagick:$(date +%s)