[extra-02] access db using env vars

To make easier to run the app locally with docker and in CI, the
configurations were adjusted to read the database access from
environment variables:

1. POSTGRES_USER
2. POSTGRES_PASS
3. POSTGRES_HOST

config/dev.exs

@@ -2,10 +2,10 @@ use Mix.Config
 
 # Configure your database
 config :wabanex, Wabanex.Repo,
-  username: "postgres",
-  password: "postgres",
+  username: System.get_env("POSTGRES_USER") || "postgres",
+  password: System.get_env("POSTGRES_PASS") || "postgres",
   database: "wabanex_dev",
-  hostname: "localhost",
+  hostname: System.get_env("POSTGRES_HOST") || "localhost",
   show_sensitive_data_on_connection_error: true,
   pool_size: 10