It took me a while to figure out how to setup postgis on a gitlab-ci build, and could not find anything on the docs, so here is a brief explanation of what I did:
First of all, I used mdillon/postgis as a base image for the service, to avoid having to install postgis on the postgres:9.4 base image over and over.
Using custom docker images as services is explained on the Using Docker Images page of the official guide.
But in short, all I had to do is make a few following changes to .gitlab-ci.yml:
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