feat: upgrade runtime and system dependencies (#5)

* Upgrade elixir, erlang, and postgres to their latest versions.
* Fix CI configuration, so tests are executed.
* Migrate project configuration from `Mix.Config` to `Config`.

Reviewed-on: #5

.drone.yml

@@ -6,13 +6,21 @@ name: test
 trigger:
   event:
     include:
-      - push
       - pull_request
 
 steps:
+  - name: database healthcheck
+    image: 'postgres:14.2-alpine'
+    environment:
+      PGUSER: postgres
+      PGPASSWORD: postgres
+      PGHOST: db
+    commands:
+      - while ! pg_isready; do sleep 1; done
   - name: test
-    image: 'elixir:1.21.1'
-    environemnt:
+    image: 'elixir:1.13.4'
+    environment:
+      MIX_ENV: test
       POSTGRES_HOST: db
       POSTGRES_USER: postgres
       POSTGRES_PASS: postgres
@@ -22,7 +30,7 @@ steps:
 
 services:
   - name: db
-    image: 'postgres:13.3-alpine'
+    image: 'postgres:14.2-alpine'
     environment:
       POSTGRES_USER: postgres
       POSTGRES_PASSWORD: postgres