Moving to ONOS 2.5.7-rc1 to incorporate persistent mastership

Change-Id: If3f7670dfa08be8507f39082e3745c4e6dc07eae
diff --git a/Dockerfile.voltha-onos b/Dockerfile.voltha-onos
index dfc5456..850bb4a 100644
--- a/Dockerfile.voltha-onos
+++ b/Dockerfile.voltha-onos
@@ -34,7 +34,7 @@
 
 
 # Unpack and install specific apps from download stage
-FROM onosproject/onos:2.5.6 as install
+FROM onosproject/onos:2.5.7-rc1 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:2.5.6
+FROM onosproject/onos:2.5.7-rc1
 
 # The ENV settings must be replicated below as they are not shared between stages
 ENV ONOS=/root/onos