tests: updated ubuntu version in Dockerfile

tests/docker-dns-srv/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:17.10
+FROM ubuntu:18.04
 
 RUN rm /bin/sh && ln -s /bin/bash /bin/sh
 RUN echo 'debconf debconf/frontend select Noninteractive' | debconf-set-selections

tests/docker-static-ip/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:17.10
+FROM ubuntu:18.04
 
 RUN rm /bin/sh && ln -s /bin/bash /bin/sh
 RUN echo 'debconf debconf/frontend select Noninteractive' | debconf-set-selections