Use database password in Docker Compose environment

Addresses docker-library/postgres#681.

INSTALL.md

@@ -37,7 +37,7 @@ There is a rudimentary docker-compose configuration for production usage (`docke
 
 
 1. Configure OSEM
-   You have at least to set `SECRET_KEY_BASE`
+   You have at least to set `OSEM_DB_PASSWORD` and `SECRET_KEY_BASE`
    ```
    cp dotenv.example .env.production
    vim .env.production

docker-compose.yml

@@ -5,6 +5,7 @@ services:
     image: postgres
     environment:
       PGDATA: /var/lib/postgresql/data/pgdata
+      POSTGRES_PASSWORD: $OSEM_DB_PASSWORD
   osem:
     build:
       context: .

docker-compose.yml.production-example

@@ -5,6 +5,7 @@ services:
     image: postgres
     environment:
       PGDATA: /var/lib/postgresql/data/pgdata
+      POSTGRES_PASSWORD: $OSEM_DB_PASSWORD
     volumes:
       - osem_production_database:/var/lib/postgresql/data/pgdata
   production_web: