Add .env.example and update docker-compose to use env_file for configuration

.env.example

@@ -0,0 +1,23 @@
+# 🌐 Frontend
+PUBLIC_SERVER_URL=http://server:8000 # Should be the service name of the backend with port 8000, even if you change the port in the backend service
+ORIGIN=http://localhost:8015
+BODY_SIZE_LIMIT=Infinity
+
+# 🐘 PostgreSQL Database
+PGHOST=db
+PGDATABASE=database
+PGUSER=adventure
+PGPASSWORD=changeme123
+
+# 🔒 Django Backend
+SECRET_KEY=changeme123
+DJANGO_ADMIN_USERNAME=admin
+DJANGO_ADMIN_PASSWORD=admin
+DJANGO_ADMIN_EMAIL=admin@example.com
+PUBLIC_URL=http://localhost:8016 # Match the outward port, used for the creation of image urls
+CSRF_TRUSTED_ORIGINS=http://localhost:8016,http://localhost:8015
+DEBUG=False
+FRONTEND_URL=http://localhost:8015 # Used for email generation. This should be the url of the frontend
+
+# 🗺️ Optional
+# GOOGLE_MAPS_API_KEY=your_google_maps_api_key # https://adventurelog.app/docs/configuration/google_maps_integration.html