[CI] use supercollider docker image
This MR modifies gitlab-ci in order to used a new base Docker image. This speeds up SATIE's CI quite a bit.
I've created an experimental image of SuperCollider + sc3-plugins: https://hub.docker.com/r/pdupuis/supercollider
This image should be hosted under the metalabsat Docker hub organization, but I don't seem to have the right credentials. For now, it lives as pdupuis/supercollider
.