Update Dockerfile.design to not depend on decidim/docker (#7077)
Update Dockerfile.design to not depend on decidim/docker (#7077)

Decouple it from decidim/decidim-generator image so it can use develop branch's ruby version
AjuntamentdeBarcelona/decidim,decidim/decidim,codegram/decidim,decidim/decidim,AjuntamentdeBarcelona/decidim,AjuntamentdeBarcelona/decidim,codegram/decidim,codegram/decidim,AjuntamentdeBarcelona/decidim,codegram/decidim,decidim/decidim