Change Docker container to bionic
Commit 117b8a45164bdb5ad2ab9e6809cbc923cffa338e requires version 6+
of i686-w64-mingw32-gcc to run the mingw builds, but Ubuntu Xenial (16.04)
supplies 5.3.1. Change the Docker container to Ubuntu Bionic (18.04) to
pick up a version that will run the tests.
diff --git a/tests/docker/xenial/Dockerfile b/tests/docker/bionic/Dockerfile
similarity index 99%
rename from tests/docker/xenial/Dockerfile
rename to tests/docker/bionic/Dockerfile
index f16ae75..1d96184 100644
--- a/tests/docker/xenial/Dockerfile
+++ b/tests/docker/bionic/Dockerfile
@@ -22,7 +22,7 @@
#
# This file is part of Mbed TLS (https://tls.mbed.org)
-FROM ubuntu:xenial
+FROM ubuntu:bionic
ARG MAKEFLAGS_PARALLEL=""