db:
    image: postgres:9.4
    ports:
        - 5432:5432
    environment:
        POSTGRES_PASSWORD: postgres
        POSTGRES_USER: postgres
        PGDATA : /var/lib/postgresql/data
    volumes:
        - /projects/own/docker_php/pgdata:/var/lib/postgresql/data

db_1  | initdb: directory "/var/lib/postgresql/data" exists but is not empty
db_1  | If you want to create a new database system, either remove or empty
db_1  | the directory "/var/lib/postgresql/data" or run initdb
db_1  | with an argument other than "/var/lib/postgresql/data".

FROM postgres:9.4
COPY pgdata /var/lib/postgresql/data

version: '2'

services:
  postgres9:
    image: postgres:9.4
    expose:
      - 5432
    volumes:
      - data:/var/lib/postgresql/data

volumes:
  data: {}

$ docker-compose up

$ docker exec -it $(docker-compose ps -q postgres9 ) psql -Upostgres -c 'z'

$ docker exec -it $(docker-compose ps -q postgres9 ) psql -Upostgres -c 'create table beer()'

$ docker exec -it $(docker-compose ps -q postgres9 ) psql -Upostgres -c 'z'

                         Access privileges
 Schema |   Name    | Type  | Access privileges | Column access privileges
--------+-----------+-------+-------------------+--------------------------
 public | beer      | table |                   |

$ docker-compose stop

$ docker-compose up

$ docker exec -it $(docker-compose ps -q postgres9 ) psql -Upostgres -c 'z'
                         Access privileges
 Schema |   Name    | Type  | Access privileges | Column access privileges
--------+-----------+-------+-------------------+--------------------------
public | beer      | table |                   |

$ docker exec -it $(docker-compose ps -q postgres9 ) pg_dump -Upostgres > backup.sql

$ docker exec -it $(docker-compose ps -q postgres9 ) psql -Upostgres -c 'drop table beer'

$ docker exec -i $(docker-compose ps -q postgres9 ) psql -Upostgres < backup.sql

$ docker exec -it $(docker-compose ps -q postgres9 ) psql -Upostgres -c 'z'
                         Access privileges
Schema |   Name    | Type  | Access privileges | Column access privileges
--------+-----------+-------+-------------------+--------------------------
public | beer      | table |                   |

postgres9:
  image: postgres:9.4
  ports:
    - 5432:5432
  volumes_from:
    - pg_data
  environment:
    POSTGRES_PASSWORD: postgres
    POSTGRES_USER: postgres
    PGDATA : /var/lib/postgresql/data/pgdata

pg_data:
  image: alpine:latest
  volumes:
    - /var/lib/postgresql/data/pgdata
  command: "true"