Add a way to opt-out of using definitely-gp for workspaces

[jeanp413]

Is your feature request related to a problem? Please describe

I was trying to open a new workspace of upstream vscode repo and got surprised that it was using some custom docker image instead of the default image, furthemore this custom image is broken and I couldn't open the workspace at all.
Later I found there's this global configuration repo definitely-gp #7278 (comment)

Describe the behaviour you'd like

Add an option (in the dashboard?) to opt-out of using definitely-gp, I don't want a custom image to avoid wasting time waiting for it to build plus it's broken currently and I can see it breaking again in the future

[svenefftinge]

will be resolved by #8473