Set up basic CI

.drone.yml

@@ -2,9 +2,17 @@ kind: pipeline
 type: docker
 name: default
 
+services:
+  - name: postgres
+    image: docker.io/library/postgres:17.2
+
 steps:
   - name: lint & test
-    image: docker.io/elixir:1.18.3-otp-27
+    image: docker.io/library/elixir:1.18.3-otp-27
     commands:
+      # Install hex package manager
+      - mix local.hex --force
+      # Check for compilation errors & warnings
       - mix compile --warnings-as-errors
+      # Check formatting
       - mix format --check-formatted