Add automated container build workflow

This workflow automatically builds and publishes Docker containers to GitHub
Container Registry (ghcr.io) when:
- Dockerfile changes are pushed
- Manually triggered via workflow_dispatch
- Monthly (1st of each month)

Benefits:
- Pre-built containers speed up CI/CD
- Containers are publicly available for all users
- Automatic monthly rebuilds keep images fresh

Author: nchapman

.github/workflows/build-container.yml

@@ -0,0 +1,46 @@
+name: Build and Publish Container
+
+on:
+  push:
+    branches: [main, master]
+    paths:
+      - 'Dockerfile'
+      - '.github/workflows/build-container.yml'
+  workflow_dispatch:
+  schedule:
+    - cron: '0 0 1 * *'  # Monthly rebuild
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: ghcr.io/${{ github.repository }}
+          tags: |
+            type=raw,value=latest
+            type=sha,prefix={{branch}}-
+
+      - name: Build and push container
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}