add configuration of the postgreSQL username in environment variables

docker/database.yml

@@ -5,7 +5,7 @@ default: &default
   adapter: postgresql
   encoding: unicode
   pool: 25
-  username: postgres
+  username: <%= ENV["POSTGRES_USERNAME"] %>
   password: <%= ENV["POSTGRES_PASSWORD"] %>
   host: <%= ENV["POSTGRES_HOST"] %>
 

setup/env.example

@@ -1,4 +1,5 @@
 POSTGRES_HOST=postgres
+POSTGRES_USERNAME=postgres
 REDIS_HOST=redis
 ELASTICSEARCH_HOST=elasticsearch