Ecto allows the use of a unix socket to connect to postgres, instead of TCP. In order to use a unix socket in a exs file, you should remove the hostname list item, and provide the socket_dir list item.
# dev.exs
...
# Configure your database
config :myapp, Myapp.Repo,
adapter: Ecto.Adapters.Postgres,
database: "myapp_dev",
socket_dir: "/var/run/postgresql",
pool_size: 10