Add GitHub Actions workflow to build and push Docker image to ghcr.io

Publishes ghcr.io/literatecomputing/mail-receiver:release on every push to main.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: pfaffman

.github/workflows/docker.yml

@@ -0,0 +1,31 @@
+name: Docker
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          tags: ghcr.io/literatecomputing/mail-receiver:release