Change docker URL sources as artifact directory was switched to ubuntu-xenial and fix builder image revision selection

Closes: #2447 (closed)
Closes: #2398 (closed)

• Change artifact directory to ubuntu-xenial from ubuntu-16.04
• Builder image revision can not be read from a file - no shell execution is performed in image: field of .gitlab-ci.yml.
  • Modify the docs so that it instructs to check the .gitlab-ci.yml file to find out the image revision
  • Update .gitlab-ci.yml to previous state
  • Delete BUILDER_IMAGE_REVISION file