Use environment variable for password in Docker

We were getting an error since we started using the postgres 9.6 image:

```
Attaching to app_1, database_1
database_1  | Error: Database is uninitialized and superuser password is not specified.
database_1  |        You must specify POSTGRES_PASSWORD to a non-empty value for the
database_1  |        superuser. For example, "-e POSTGRES_PASSWORD=password" on "docker run".
database_1  |
database_1  |        You may also use "POSTGRES_HOST_AUTH_METHOD=trust" to allow all
database_1  |        connections without a password. This is *not* recommended.
database_1  |
database_1  |        See PostgreSQL documentation about "trust":
database_1  |        https://www.postgresql.org/docs/current/auth-trust.html
```

docker-compose.yml

@@ -6,7 +6,8 @@ services:
     # use the preferred version of the official Postgres image
     # see https://hub.docker.com/_/postgres/
     image: postgres:9.6.21
-
+    environment:
+      - POSTGRES_PASSWORD=$POSTGRES_PASSWORD
     # persist the database between containers by storing it in a volume
     volumes:
       - docker-example-postgres:/var/lib/postgresql/data
@@ -37,6 +38,7 @@ services:
       - bundle:/usr/local/bundle:delegated
       - "$SSH_AUTH_SOCK:/tmp/agent.sock"
     environment:
+      - POSTGRES_PASSWORD=$POSTGRES_PASSWORD
       - SSH_AUTH_SOCK=/tmp/agent.sock
 volumes:
   docker-example-postgres: {}