dev.secret.exs ignored

I followed the installation instructions in the README. I created config/dev.secret.exs and copied the database section in there, updating the username and password as specified.

I then tried mix ecto.create and got this error:

** (Mix) The database for Pleroma.Repo couldn't be created: FATAL 28P01 (invalid_password): password authentication failed for user "postgres"

07:35:08.711 [error] GenServer #PID<0.219.0> terminating
** (Postgrex.Error) FATAL 28P01 (invalid_password): password authentication failed for user "postgres"
    (db_connection) lib/db_connection/connection.ex:148: DBConnection.Connection.connect/2
    (connection) lib/connection.ex:622: Connection.enter_connect/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: nil
State: Postgrex.Protocol

It seems to me that it's ignoring the dev.secret.exs file and only using the database section in dev.exs (or somewhere else?).

Note that I'm using postgres 10 because that's what my system ended up installing for me.

dev.secret.exs contains:

# Configure your database
config :pleroma, Pleroma.Repo,
  adapter: Ecto.Adapters.Postgres,
  username: "pleroma",
  password: "a_super_secret_password",
  database: "pleroma_dev",
  hostname: "localhost",
  pool_size: 10

Any ideas?