Build and publish docker image from main branch (#227)

.github/workflows/docker.yml

@@ -0,0 +1,26 @@
+name: Build and publish docker image
+
+on:
+  push:
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+    name: Build and publish docker image
+    runs-on: ubuntu-latest
+    steps:
+      -
+        name: Login to GitHub Container Registry
+        uses: docker/login-action@v2
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      -
+        name: Build and push
+        uses: docker/build-push-action@v3
+        with:
+          push: ${{ github.ref == 'refs/heads/main' }}
+          tags: ghcr.io/matrix-org/matrix-authentication-service:sha-${{ github.sha }}