* 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.
Enable running tests under Docker. This makes it easier to spin up an
environment with all dependencies (especially the multiple versions of
openssl and gnutls needed).
* tests/docker/xenial/Dockerfile: Definition for the docker image,
including local builds for openssl and gnutls.
* tests/scripts/docker_env.sh: New helper script that creates the Docker
image and has a function to run a command in the Docker container.
* tests/docker/all-in-docker.sh: Wrapper for all.sh under Docker.
* tests/docker/basic-in-docker.sh: Script that runs the same commands as
.travis.yml, in Docker.
* tests/ssl-opt-in-docker.sh: Wrapper to run ssl-opt.sh in Docker.
* tests/compat-in-docker.sh: Wrapper to run compat.sh in Docker.
* tests/make-in-docker.sh: Wrapper to run make in Docker.
Change-Id: Ie092b1deed24c24c3859754535589523ce1d0a58
