Improve compatibility with firewalled networks
* Allow specifying MBEDTLS_DOCKER_REGISTRY for organizations that have
a mirroring proxy of Docker Hub
* Specify "--network host" during build to ensure use of the host's
DNS resolution.
diff --git a/tests/docker/bionic/Dockerfile b/tests/docker/bionic/Dockerfile
index 948e506..1b2d40f 100644
--- a/tests/docker/bionic/Dockerfile
+++ b/tests/docker/bionic/Dockerfile
@@ -21,10 +21,11 @@
# limitations under the License.
#
# This file is part of Mbed TLS (https://tls.mbed.org)
-
-FROM ubuntu:bionic
-
ARG MAKEFLAGS_PARALLEL=""
+ARG MY_REGISTRY=
+
+FROM ${MY_REGISTRY}ubuntu:bionic
+
ENV DEBIAN_FRONTEND noninteractive