CORD-1134 Add :candidate to Chameleon Dockerfile, remove unused Dockerfile.devel

Change-Id: I06017305ffbcb83d699f2996863edd4faec2e10c
diff --git a/containers/chameleon/Dockerfile.chameleon b/containers/chameleon/Dockerfile.chameleon
index e00bc5e..f5fb70c 100644
--- a/containers/chameleon/Dockerfile.chameleon
+++ b/containers/chameleon/Dockerfile.chameleon
@@ -1,5 +1,5 @@
 # xosproject/chameleon
-FROM xosproject/xos-base
+FROM xosproject/xos-base:candidate
 
 # xos-base already has protoc and dependencies installed