* 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.
Commit 117b8a4516 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.