fix: respect NODE_DB_PATH environment variable in Docker (fixes #40)

- Fixed docker-entrypoint.sh to use NODE_DB_PATH instead of hardcoded paths
- Added log_message() helper to prevent stdio mode output corruption
- Fixed directory creation race condition with proper ownership
- Added path validation to ensure NODE_DB_PATH ends with .db
- Updated rebuild.ts to respect NODE_DB_PATH environment variable
- Added comprehensive documentation for custom database paths
- Bumped version to 2.7.16

The bug was caused by hardcoded /app/data/nodes.db paths in the Docker
entrypoint script that ignored the NODE_DB_PATH environment variable.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/scripts/rebuild.ts

@@ -15,7 +15,8 @@ import * as path from 'path';
 async function rebuild() {
   console.log('🔄 Rebuilding n8n node database...\n');
   
-  const db = await createDatabaseAdapter('./data/nodes.db');
+  const dbPath = process.env.NODE_DB_PATH || './data/nodes.db';
+  const db = await createDatabaseAdapter(dbPath);
   const loader = new N8nNodeLoader();
   const parser = new NodeParser();
   const mapper = new DocsMapper();