Add variable to configure the controller and invoker image prefix (#69)

diff --git a/docker-compose/README.md b/docker-compose/README.md
index 4195343..42f68c2 100644
--- a/docker-compose/README.md
+++ b/docker-compose/README.md
@@ -144,3 +144,9 @@
 - `DOCKER_IMAGE_PREFIX` - specify a custom image prefix. I.e. ```DOCKER_IMAGE_PREFIX=my-prefix make quick-start```
 
 These 2 variable allow you to execute a JS action using the container `registry.example.com/my-prefix/nodejs6action`.
+
+## Local Docker containers for controllers and invokers
+
+By default this setup uses published images for controller and invokers from `openwhisk` namespace i.e. 
+`openwhisk/controller` and `openwhisk/invoker`. To make use of locally build images you can use `DOCKER_OW_IMAGE_PREFIX`
+variable i.e. `DOCKER_OW_IMAGE_PREFIX=whisk make quick-start`
\ No newline at end of file
diff --git a/docker-compose/docker-compose.yml b/docker-compose/docker-compose.yml
index 28249de..d268d3d 100644
--- a/docker-compose/docker-compose.yml
+++ b/docker-compose/docker-compose.yml
@@ -37,7 +37,7 @@
 
   # WHISK CONTROLLER
   controller:
-    image: openwhisk/controller
+    image: ${DOCKER_OW_IMAGE_PREFIX:-openwhisk}/controller
     command: /bin/sh -c "controller/bin/controller 0 >> /logs/controller-local_logs.log 2>&1"
     links:
       - db:db.docker
@@ -76,7 +76,7 @@
 
   # WHISK INVOKER AGENT
   invoker:
-    image: openwhisk/invoker
+    image: ${DOCKER_OW_IMAGE_PREFIX:-openwhisk}/invoker
     command: /bin/sh -c "/invoker/bin/invoker 0 >> /logs/invoker-local_logs.log 2>&1"
     privileged: true
     pid: "host"