docs: add 1Panel Docker production deploy stack

Add Dockerfile.prod, docker-compose.prod.yml, OpenResty example,
and migration plan for Baota to 1Panel + Compose deployment.

docker/.env.prod.example

@@ -0,0 +1,17 @@
+# Production Compose env — copy to docker/.env.prod (never commit)
+# Migrate NEXUS_* secrets from Baota /www/wwwroot/.../.env UNCHANGED.
+
+MYSQL_ROOT_PASSWORD=change_me_root_strong
+MYSQL_PASSWORD=change_me_nexus_strong
+
+# Required — copy from production .env (do NOT regenerate)
+NEXUS_SECRET_KEY=
+NEXUS_API_KEY=
+NEXUS_ENCRYPTION_KEY=
+
+# Public URL
+NEXUS_CORS_ORIGINS=https://api.synaglobal.vip
+NEXUS_API_BASE_URL=https://api.synaglobal.vip
+
+# Optional
+# NEXUS_PUBLISH_PORT=8600