ci: add Docker Hub publish workflow

Builds and pushes tdurieux/anonymous_github to Docker Hub on push to
main, v* tags, and manual dispatch. Multi-arch (amd64/arm64) with GHA
layer caching. Closes #478.

.github/workflows/docker-publish.yml

@@ -0,0 +1,45 @@
+name: Publish Docker image
+
+on:
+  push:
+    branches: [main]
+    tags: ['v*']
+  workflow_dispatch:
+
+jobs:
+  build-and-push:
+    name: Build and push image
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: docker/setup-qemu-action@v3
+
+      - uses: docker/setup-buildx-action@v3
+
+      - uses: docker/login-action@v3
+        with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+      - id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: tdurieux/anonymous_github
+          tags: |
+            type=raw,value=v2,enable={{is_default_branch}}
+            type=raw,value=latest,enable={{is_default_branch}}
+            type=ref,event=tag
+            type=sha,prefix=sha-,enable={{is_default_branch}}
+
+      - uses: docker/build-push-action@v6
+        with:
+          context: .
+          push: true
+          platforms: linux/amd64,linux/arm64
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max