Add Swarm documentation (#405)

Workaround described in #236

Author: JeromeGillard

docs/docker-compose.md

@@ -34,6 +34,42 @@ You can now run your tests by using the `--seleniumUrl="http://localhost:4444/wd
 However if your web application under test is running in localhost, e.g. `--appHost=localhost` or is publicly not accessible
 you should instead either dockerize your application as shown in the example [adwords_mock](https://github.com/elgalu/google_adwords_mock) within the [docker-compose-tests.yml][]. Its there shown as another [service](https://docs.docker.com/compose/compose-file/#/service-configuration-reference). Another option (if you don't want to dockerize your app yet) is to somehow replace `--appHost=localhost` with `--appHost=d.host.loc.dev` in the config file of your testing framework. The string `d.host.loc.dev` is a place holder inside the docker container that points to the IP of your localhost.
 
+## Docker Swarm mode
+You might get the following error if you try to run a `SELENIUM_NODE` as part of a Swarm stack:
+```
+$ docker stack deploy selenium
+Updating service selenium_selenium-chrome (id: joru9fw8xej2imcrvgblcrc85)
+failed to create service selenium_selenium-chrome: Error response from daemon: rpc error: code = InvalidArgument desc = expanding env failed: expanding env "SELENIUM_NODE_HOST={{CONTAINER_IP}}": template: expansion:1: function "CONTAINER_IP" not defined
+```
+Thanks to https://github.com/elgalu/docker-selenium/issues/236 you just have to update your variable `SELENIUM_NODE_HOST` to `{{__CONTAINER_IP__}}`.  
+Example in a `docker-compose file`:
+```
+selenium-firefox:
+    image: elgalu/selenium
+    depends_on:
+      - hub
+    volumes:
+      - /dev/shm:/dev/shm
+    deploy:
+      replicas: 3
+    environment:
+      - DEBUG=true
+      - PICK_ALL_RANDOM_PORTS=true
+      - SELENIUM_HUB_HOST=hub
+      - SELENIUM_HUB_PORT=4444
+      - SELENIUM_NODE_HOST={{__CONTAINER_IP__}}
+      - SCREEN_WIDTH=1300
+      - SCREEN_HEIGHT=999
+      - VIDEO=false
+      - AUDIO=false
+      - GRID=false
+      - CHROME=false
+      - FIREFOX=true
+      - MAX_INSTANCES=1
+      - MAX_SESSIONS=1
+```
+
+
 ### Cleanup
 Once your tests are done you can clean up: