Investigate how to speed up asset-precompile

Our asset precompile takes a long time to run, 786s in one of the latest builds.

We should research if we can find a way to speed this up.

The asset:precompile task should be smart enough to only compile assets that changed. However, we purge the whole gitlab-rails definition and thus loose the previous state.

I think we should look into adding a way of packing those assets as cache which would be reused in the next build.

Maybe splitting asset compilation into a separate definition could help?

/cc @gitlab-build-team