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: I8464d3ca415a24f407aebd142cd317863eeb6c35
diff --git a/Dockerfile b/Dockerfile
index 5b0bf95..de76cd8 100755
--- a/Dockerfile
+++ b/Dockerfile
@@ -15,7 +15,7 @@
 
 #----------------------------------------------------------
 
-FROM nexus3.o-ran-sc.org:10004/bldr-ubuntu18-c-go:3-u18.04-nng AS appmgr-build
+FROM nexus3.o-ran-sc.org:10004/bldr-ubuntu18-c-go:4-u18.04-nng AS appmgr-build
 
 RUN apt-get update -y && apt-get install -y jq