Squash docker images after build

[Dmitry Kaigarodеsev]

We need to squash docker images on infra-ci after build
for avoiding lack of space on related Jenkins slave:

https://infra-ci.fuel-infra.org/label/docker-builder/

root@infra-ci-slave01:/home/jenkins# du -hs /var/lib/docker/aufs/*
306G /var/lib/docker/aufs/diff
26M /var/lib/docker/aufs/layers
26M /var/lib/docker/aufs/mnt

Probably this tool may be useful:
https://github.com/jwilder/docker-squash

[Pawel Brzozowski (pbrzozowski)]

@Dmitry can we just delete these images after building?

[Dmitry Kaigarodеsev (dkaiharodsev)]

@Pawel, yes, we can - since we'll upload it to the 'docker registry' after test,
but we also should be sure that we'll remove unneeded parent images,

so my main idea was like this:
- squash all tree of images into 1 image
- test it
- remove it