docs: demonstrate use of environment variables in config (#571)

* docs: demonstrate use of environment variables in config

* chore: reorder env vars to match order used in DB connection

.examples/docker-compose-postgres-storage/config/config.yaml

@@ -1,6 +1,6 @@
 storage:
   type: postgres
-  path: "postgres://username:password@postgres:5432/gatus?sslmode=disable"
+  path: "postgres://${POSTGRES_USER}:${POSTGRES_PASSWORD}@postgres:5432/${POSTGRES_DB}?sslmode=disable"
 
 endpoints:
   - name: back-end

.examples/docker-compose-postgres-storage/docker-compose.yml

@@ -18,6 +18,10 @@ services:
     restart: always
     ports:
       - "8080:8080"
+    environment:
+      - POSTGRES_USER=username
+      - POSTGRES_PASSWORD=password
+      - POSTGRES_DB=gatus
     volumes:
       - ./config:/config
     networks: