Cache Python packages in CI Docker runs

[dshemetov]

You can cache python packages between Docker runs like this

RUN --mount=type=cache,target=/root/.cache/pip pip install -r requirements.txt

This requires the BuildKit functionality, which requires Docker >= 18.09 and can be activated with the environment variable DOCKER_BUILDKIT=1 docker build ..

If this is possible in CI, which it probably is, it could speed up CI runs as well as local image builds.