Create automated build workflow

.github/workflows/build.yml

@@ -0,0 +1,18 @@
+name: Build Docker Image
+on:
+  push:
+    branches: master
+
+jobs:
+  build:
+    name: Push to GHCR
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Login
+      run: echo ${{ secrets.GHCR_PAT }} | docker login ghcr.io -u dchidell --password-stdin
+    - name: Build
+      run: docker build . --file Dockerfile --tag ghcr.io/dchidell/dnsmasq-web:latest
+    - name: Push
+      run: |
+        docker push ghcr.io/dchidell/dnsmasq-web:latest