AETHER-869 Upgrade base OS version

Change-Id: I620752299c105970e8aa43cfb0d68e92f8a61d98
diff --git a/Dockerfile.base b/Dockerfile.base
index 4954883..25159a0 100644
--- a/Dockerfile.base
+++ b/Dockerfile.base
@@ -13,7 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM ubuntu:16.04 AS oai-base
+FROM ubuntu:18.04 AS oai-base
+ENV DEBIAN_FRONTEND noninteractive
 RUN apt-get update && apt-get install -y \
     sudo \
     vim-common \