services:
  db:
    image: postgres:13.2
    container_name: db
    ports:
      - "6021:5432"
    volumes:
      - ../pg:/var/lib/postgresql/data/pgdata
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password
      PGDATA: /var/lib/postgresql/data/pgdata

db | initdb: error: directory "/var/lib/postgresql/data" exists but is not empty
db | If you want to create a new database system, either remove or empty
db | the directory "/var/lib/postgresql/data" or run initdb
db | with an argument other than "/var/lib/postgresql/data".

 SELECT column_name INTO var_name FROM table_name WHERE ...