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/scripts/docker_env.sh b/tests/scripts/docker_env.sh
index 6b65b03..04efa82 100755
--- a/tests/scripts/docker_env.sh
+++ b/tests/scripts/docker_env.sh
@@ -38,7 +38,7 @@
# default values, can be overridden by the environment
-: ${MBEDTLS_DOCKER_GUEST:=xenial}
+: ${MBEDTLS_DOCKER_GUEST:=bionic}
DOCKER_IMAGE_TAG="armmbed/mbedtls-test:${MBEDTLS_DOCKER_GUEST}"
