[autodevops] use GITLAB_ENVIRONMENT_URL as CW_BASE_URL
When deploying a cube using the AutoDevops pipeline the base environment contains $GITLAB_ENVIRONMENT_URL which gives the URL being served by the review app.
Maybe this environment variable can be picked up by cubicweb and used instead of CW_BASE_URL ?
Not sure if this the right place to do this, so I'll open an issue on the cubicweb-docker project too, where I guess we could "translate" gitlab/heptapod specific variables to cubicweb specific variables.