chore: Add database config for CI

.gitlab-ci.yml

@@ -1,4 +1,10 @@
 image: "elixir:1.10"
 
+variables:
+  DATABASE_URL: "postgres"
+
+services: 
+- name: postgres:12
+
 tests:
   script: script/cibuild

config/test.exs

@@ -9,7 +9,7 @@ config :legendary, Legendary.Repo,
   username: "postgres",
   password: "postgres",
   database: "legendary_test#{System.get_env("MIX_TEST_PARTITION")}",
-  hostname: "localhost",
+  hostname: System.get_env("DATABASE_URL") || "localhost",
   pool: Ecto.Adapters.SQL.Sandbox
 
 # We don't run a server during test. If one is required,