After modernising docker configuration
(I1215e04ec15b01c0b43bac8c0e81293f6724f278), we lost our
registry-mirrors configuration in CI that lets us use a mirror of
Dockerhub.

This change uses the new docker_custom_config variable to configure the
registry mirror.

Change-Id: I1430413c12e9d0b59e4f216ff66372de0f3a4f21