Upgrade Dockerfile base build image

Use bldr-ubuntu18-c-go:4-u18.04-nng

Signed-off-by: Lott, Christopher (cl778h) <cl778h@att.com>
Change-Id: I468a2e23ee5e09eb75f8891d457c525d81a44233
diff --git a/ci/Dockerfile b/ci/Dockerfile
index 73ec7cd..f2fc04e 100755
--- a/ci/Dockerfile
+++ b/ci/Dockerfile
@@ -17,7 +17,7 @@
 #
 #----------------------------------------------------------
 
-FROM nexus3.o-ran-sc.org:10004/bldr-ubuntu18-c-go:3-u18.04-nng as xapp-base
+FROM nexus3.o-ran-sc.org:10004/bldr-ubuntu18-c-go:4-u18.04-nng as xapp-base
 
 RUN apt-get update -y \
     &&apt-get install -y \