Hugo extended version support

Currently CI builds using the registry.gitlab.com/pages/hugo:latest image fail if they require features that are only available in the Hugo extended version.

Would it be possible to use hugo_extended_${HUGO_VERSION}_Linux-64bit.tar.gz instead of hugo_${HUGO_VERSION}_Linux-64bit.tar.gz in the Dockerfile?

I am happy to make a pull request for this if needed.

Example output when attempting to use a theme that requires Hugo extended features:

Running with gitlab-runner 11.10.0-rc2 (10efa505)
  on docker-auto-scale fa6cab46
Using Docker executor with image registry.gitlab.com/pages/hugo:latest ...
Pulling docker image registry.gitlab.com/pages/hugo:latest ...
Using docker image sha256:9eb5e5f1ff5f999c87997da80cddb50d1354d9a7cd7e2bb60548a8c982b62600 for registry.gitlab.com/pages/hugo:latest ...
Running on runner-fa6cab46-project-12022452-concurrent-0 via runner-fa6cab46-srm-1556175271-450b8a82...
Initialized empty Git repository in /builds/alexwootton/personal-wishlist/.git/
Fetching changes...
Created fresh repository.
From https://gitlab.com/alexwootton/personal-wishlist
 * [new branch]      master     -> origin/master
Checking out c4096083 as master...

Updating/initializing submodules recursively...
Submodule 'themes/hugo-whisper-theme' (https://github.com/jugglerx/hugo-whisper-theme.git) registered for path 'themes/hugo-whisper-theme'
Cloning into '/builds/alexwootton/personal-wishlist/themes/hugo-whisper-theme'...
Submodule path 'themes/hugo-whisper-theme': checked out 'c679e0b497ad533a75fb4374d602cff91898b773'
Entering 'themes/hugo-whisper-theme'
$ hugo
Building sites … ERROR 2019/04/25 06:55:36 error: failed to transform resource: TOCSS: failed to transform "scss/style.scss" (text/x-scss): this feature is not available in your current Hugo version, see https://goo.gl/YMrWcn for more information
Total in 20 ms
Error: Error building site: logged 1 error(s)
ERROR: Job failed: exit code 255