docs(examples): update docker-compose.yml (socketio#4594)

Updated docker compose to be in line with current practices.

Reference: https://docs.docker.com/compose/

examples/cluster-nginx/docker-compose.yml

@@ -1,58 +1,43 @@
+services:
+  nginx:
+    image: nginx:alpine
+    volumes:
+      - ./nginx.conf:/etc/nginx/nginx.conf:ro
+    ports:
+    - "3000:80"
 
-nginx:
-  image: nginx:alpine
-  volumes:
-    - ./nginx.conf:/etc/nginx/nginx.conf:ro
-  links:
-    - server-john
-    - server-paul
-    - server-george
-    - server-ringo
-  ports:
-   - "3000:80"
-
-server-john:
-  build: ./server
-  links:
-    - redis
-  expose:
+  server-john:
+    build: ./server
+    expose:
     - "3000"
-  environment:
-    - NAME=John
+    environment:
+      - NAME=John
 
-server-paul:
-  build: ./server
-  links:
-    - redis
-  expose:
-    - "3000"
-  environment:
-    - NAME=Paul
+  server-paul:
+    build: ./server
+    expose:
+      - "3000"
+    environment:
+      - NAME=Paul
 
-server-george:
-  build: ./server
-  links:
-    - redis
-  expose:
-    - "3000"
-  environment:
-    - NAME=George
+  server-george:
+    build: ./server
+    expose:
+      - "3000"
+    environment:
+      - NAME=George
 
-server-ringo:
-  build: ./server
-  links:
-    - redis
-  expose:
-    - "3000"
-  environment:
-    - NAME=Ringo
+  server-ringo:
+    build: ./server
+    expose:
+      - "3000"
+    environment:
+      - NAME=Ringo
 
-client:
-  build: ./client
-  links:
-    - nginx
+  client:
+    build: ./client
 
-redis:
-  image: redis:alpine
-  expose:
-    - "6379"
+  redis:
+    image: redis:alpine
+    expose:
+      - "6379"