Faster ci

• use docker socket on the ci host instead of dind to enable docker cache (saves ~4 mins)
• build from base image with yarn offline mirror (saves ~1 min)
• gitlab-runner-docker-cleanup is now running on the runner host, which should prevent the host running out of space

Base image is here: https://gitlab.coko.foundation/pubsweet/infra/tree/master/images/xpub. NO LONGER USES ALPINE IN ORDER TO AVERT SEGFAULTS DURING PUBSWEET BUILD (image size 0.821 gb --> 1.23 gb)

Runner config and docker-cleanup command are here: https://gitlab.coko.foundation/pubsweet/infra/tree/master/runners/jure-2gb-fra1-01

Closes #82 (closed)