Document usage of `DOCKER_DRIVER: overlay`

Many people are strugling with slow docker:dind. This is due to the fact that by default docker user vfs storage driver which copies the filesystem on every run. This is very disk intensive operation.

We could advise users to:

1. use recent kernel >= 4.2 ,

2. make sure that their kernel does have overlay loaded: grep overlay /proc/filesystems,

3. use in .gitlab-ci.yml:

   variables:
     DOCKER_DRIVER: overlay