Using ONOS 2.2.7-b4 as base image

Change-Id: I38a5bfa10e0ec926a078be94db903b96e2be1841
diff --git a/Dockerfile.voltha-onos b/Dockerfile.voltha-onos
index 1987991..2953760 100644
--- a/Dockerfile.voltha-onos
+++ b/Dockerfile.voltha-onos
@@ -34,7 +34,7 @@
 
 
 # Unpack and install specific apps from download stage
-FROM onosproject/onos:2.2-latest as install
+FROM onosproject/onos:2.2.7-b4 as install
 
 # The ENV settings must be replicated below as they are not shared between stages
 ENV ONOS=/root/onos
@@ -58,7 +58,7 @@
 
 
 # Create the final image coping over the installed applications from the install stage
-FROM onosproject/onos:2.2-latest
+FROM onosproject/onos:2.2.7-b4
 
 # The ENV settings must be replicated below as they are not shared between stages
 ENV ONOS=/root/onos