feat: self-hosted Docker image with Gitea Actions and Unraid docs

- Dockerfile: replace s6overlay with multi-stage build (golang:1.22-alpine
  builder + alpine:3.19 runtime), compiles from source, no goreleaser needed
- docker/entrypoint.sh: PUID/PGID support via su-exec
- .gitea/workflows/docker.yml: auto-build and push to Gitea container
  registry on every master commit (linux/amd64 + linux/arm64)
- README: add tmdb api_key docs, update Docker section to point to
  forge.dilain.com registry, add Unraid setup section

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

.gitea/workflows/docker.yml

@@ -0,0 +1,39 @@
+name: Docker
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  docker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v3
+
+      - name: qemu
+        uses: docker/setup-qemu-action@v2
+
+      - name: buildx
+        uses: docker/setup-buildx-action@v2
+
+      - name: login
+        uses: docker/login-action@v2
+        with:
+          registry: forge.dilain.com
+          username: ${{ github.actor }}
+          password: ${{ github.token }}
+
+      - name: build and push
+        uses: docker/build-push-action@v3
+        with:
+          context: .
+          file: ./docker/Dockerfile
+          platforms: linux/amd64,linux/arm64
+          push: true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          tags: |
+            forge.dilain.com/laurent/nabarr:master
+            forge.dilain.com/laurent/nabarr:latest