Implement sitebuilder image override for PaaS use case
Related to !12 (merged)
PaaS site admins should be able to use their own composer project as basis for the site, so they should be allowed to override the sitebuilder-base with sitebuilderImage:= spec.environment.imageOverride.sitebuilder.
In this case, sitebuilderImage should be used in place of the project's sitebuilder to build the nginx & php images.