docker: documents how to use master version in examples (#3656)

Signed-off-by: Adrian Cole <adrian@tetrate.io>

docker/examples/README.md

@@ -7,7 +7,10 @@ configuration requires docker-compose 1.6.0+ and docker-engine 1.10.0+.
 To start the default docker-compose configuration, run:
 
 ```bash
+# To use the last released version of zipkin
 $ docker-compose up
+# To use the last built version of zipkin
+$ TAG=master docker-compose up
 ```
 
 View the web UI at $(docker ip):9411. Traces are stored in memory.
@@ -115,7 +118,7 @@ $ docker-compose -f docker-compose-rabbitmq.yml up
 Then configure the [RabbitMQ sender](https://github.com/openzipkin/zipkin-reporter-java/blob/master/amqp-client/src/main/java/zipkin2/reporter/amqp/RabbitMQSender.java)
 using a `host` value of `localhost` or a non-local hostname if in docker.
 
-# Example
+## Example
 
 The docker-compose configuration can be extended to host an [example application](https://github.com/openzipkin/brave-example)
 using the `docker-compose-example.yml` file. That file employs [docker-compose overrides](https://docs.docker.com/compose/extends/#multiple-compose-files)