The Docker daemon will use the
unpigz tool, if it’s in the path, to extract the pulled images more efficiently than the default and slow Go based gzip implementation. See https://github.com/moby/moby/pull/35697 for further details.
As for now I’m using the following before_script:
wget http://mirrors.kernel.org/ubuntu/pool/universe/p/pigz/pigz_2.4-1_amd64.deb && sudo dpkg -i pigz_2.4-1_amd64.deb && sudo service docker restart