Basic Integration Test for backend

This patch implements a very basic test for commits and pull requests to
be run on GitHub Actions. This does not yet check much, but would have
caught something like the recent syntax error.

.github/.docker-compose-database.yml

@@ -0,0 +1,16 @@
+services:
+  db:
+    image: postgis/postgis:15-3.3
+    container_name: adventurelog-db
+    restart: unless-stopped
+    ports:
+      - "127.0.0.1:5432:5432"
+    environment:
+      POSTGRES_DB: database
+      POSTGRES_USER: adventure
+      POSTGRES_PASSWORD: changeme123
+    volumes:
+      - postgres_data:/var/lib/postgresql/data/
+
+volumes:
+  postgres_data: