feat: add Forgejo Action workflow to build and publish Docker image

Add a Forgejo Actions workflow (docker-publish.yml) that builds the
mailship Docker image on every merge to main and publishes it to the
Forgejo container registry at forgejo.lorentz.is/matt/mailship.

Tags pushed: :latest and :<commit-sha>

Serves the README's self-hosting path (docker pull) and future
pull-based deploys.

.forgejo/workflows/docker-publish.yml

@@ -0,0 +1,36 @@
+# Forgejo Action — build + publish Docker image to Forgejo container registry on merge to main
+---
+name: Docker
+
+on:
+  push:
+    branches: [main]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to Forgejo Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: forgejo.lorentz.is
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: |
+            forgejo.lorentz.is/matt/mailship:${{ github.sha }}
+            forgejo.lorentz.is/matt/mailship:latest