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>

docker/entrypoint.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+set -e
+
+PUID=${PUID:-1000}
+PGID=${PGID:-1000}
+
+if [ "$(id -u)" = "0" ]; then
+    addgroup -g "$PGID" nabarr 2>/dev/null || true
+    adduser -u "$PUID" -G nabarr -s /bin/sh -D nabarr 2>/dev/null || true
+    mkdir -p /config
+    chown -R nabarr:nabarr /config
+    exec su-exec nabarr /app/nabarr "$@"
+else
+    exec /app/nabarr "$@"
+fi