slow image builds of tests/Dockerfile

The latest release docker-py 4.0.2 has a tests/Dockerfile that is really slow to build because it is based on FROM python:3.6-jessie. This slows down moby/moby PRs that depend on building the image.

Would like to request a docker-py 4.0.3 release with the newer tests/Dockerfile. It would reduce the apt install time from 48 seconds down to 5 seconds.

With the docker-py 4.0.2 Dockerfile

$ time docker run --rm python:3.6-jessie bash -c 'apt update && apt-get -y install gnupg2 pass curl'
...
real    0m48.195s
user    0m0.042s
sys     0m0.042s

With the docker-py master (b2a1b03) Dockerfile

$ time docker run --rm python:3.7 bash -c 'apt update && apt-get -y install gnupg2 pass curl'
...
real    0m5.897s
user    0m0.046s
sys     0m0.023s

Start with tests/Dockerfile and compare the docker-py 4.0.2 version with the master revision b2a1b03 linked in the issue. Run the two documented Docker timing commands to confirm the build difference, then check the project's release process and verify that the intended Dockerfile is included in the next release.