Fix: expose MAX_TREE_DEPTH in env (#23743)

Co-authored-by: crazywoola <427733928@qq.com>

docker/.env.example

 # Allow rendering unsafe URLs which have "data:" scheme.
 ALLOW_UNSAFE_DATA_SCHEME=false
 
+# Maximum number of tree depth in the workflow
+MAX_TREE_DEPTH=50
+
 # ------------------------------
 # Environment Variables for db Service
 # ------------------------------

docker/docker-compose.yaml

   MAX_ITERATIONS_NUM: ${MAX_ITERATIONS_NUM:-99}
   TEXT_GENERATION_TIMEOUT_MS: ${TEXT_GENERATION_TIMEOUT_MS:-60000}
   ALLOW_UNSAFE_DATA_SCHEME: ${ALLOW_UNSAFE_DATA_SCHEME:-false}
+  MAX_TREE_DEPTH: ${MAX_TREE_DEPTH:-50}
   POSTGRES_USER: ${POSTGRES_USER:-${DB_USERNAME}}
   POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-${DB_PASSWORD}}
   POSTGRES_DB: ${POSTGRES_DB:-${DB_DATABASE}}