fix: remove redundant PG_USER (#21162)

docker/.env.example

 # Environment Variables for db Service
 # ------------------------------
 
-PGUSER=${DB_USERNAME}
 # The name of the default postgres user.
 POSTGRES_USER=${DB_USERNAME}
 # The password for the default postgres user.

docker/docker-compose-template.yaml

     image: postgres:15-alpine
     restart: always
     environment:
-      PGUSER: ${PGUSER:-postgres}
       POSTGRES_USER: ${POSTGRES_USER:-postgres}
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-difyai123456}
       POSTGRES_DB: ${POSTGRES_DB:-dify}

docker/docker-compose.yaml

   MAX_PARALLEL_LIMIT: ${MAX_PARALLEL_LIMIT:-10}
   MAX_ITERATIONS_NUM: ${MAX_ITERATIONS_NUM:-99}
   TEXT_GENERATION_TIMEOUT_MS: ${TEXT_GENERATION_TIMEOUT_MS:-60000}
-  PGUSER: ${PGUSER:-${DB_USERNAME}}
   POSTGRES_USER: ${POSTGRES_USER:-${DB_USERNAME}}
   POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-${DB_PASSWORD}}
   POSTGRES_DB: ${POSTGRES_DB:-${DB_DATABASE}}
     image: postgres:15-alpine
     restart: always
     environment:
-      PGUSER: ${PGUSER:-postgres}
       POSTGRES_USER: ${POSTGRES_USER:-postgres}
       POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-difyai123456}
       POSTGRES_DB: ${POSTGRES_DB:-dify}

docker/middleware.env.example

 # ------------------------------
 # Environment Variables for db Service
 # ------------------------------
-PGUSER=postgres
+POSTGRES_USER=postgres
 # The password for the default postgres user.
 POSTGRES_PASSWORD=difyai123456
 # The name of the default postgres database.