Updating Dockerfile for onos-1.5

Change-Id: I4bcaca3ebb6cd5c4ef97a4592d28f85efd01bba5
diff --git a/tools/build/docker/Dockerfile b/tools/build/docker/Dockerfile
index ef9533e..41210a2 100644
--- a/tools/build/docker/Dockerfile
+++ b/tools/build/docker/Dockerfile
@@ -21,7 +21,7 @@
 #Download and Build ONOS
 WORKDIR /src
 RUN     apt-get update && apt-get install -y python maven git curl oracle-java8-installer oracle-java8-set-default && \
-        git clone https://github.com/opennetworkinglab/onos.git && cd onos && \
+        git clone https://github.com/opennetworkinglab/onos.git && cd onos && git checkout onos-1.5 && \
         mkdir -p /root/Downloads && \
         mvn clean install && \
         tools/build/onos-package && \