[BUG #6849] let build-core-min and build-extras-min extend from bild-core/build-extras

- there are a lot of build-<something> and build-<something>-min jobs in
  module.json
- it would appear natural that the *-min job should extend from the corresonding
  build-<something> job
- that would allow to reuse all the settings from the basic job, just needing to
  add compression
- but tl;dp
fixed root-overstepping image URLs

- some image urls would overstep the server root (which didn't hurt in
  production as the web server normalized them, i guess)