AETHER-869 Upgrade base OS version

Change-Id: I620752299c105970e8aa43cfb0d68e92f8a61d98
diff --git a/Dockerfile.enb b/Dockerfile.enb
index dc3503e..99d117e 100644
--- a/Dockerfile.enb
+++ b/Dockerfile.enb
@@ -20,7 +20,7 @@
 ENV USER=root
 RUN /bin/bash -c "source oaienv" && cd cmake_targets && ./build_oai --eNB -t ETHERNET -c
 
-FROM ubuntu:16.04 AS lte-softmodem
+FROM ubuntu:18.04 AS lte-softmodem
 RUN apt-get update && apt-get install -y \
     libssl1.0.0 \
     libnettle6 \