Update documentation on building package to specify steps needed to use docker image with compiled assets from registry
In https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/build/prepare-build-environment.md step 5, we mention the asset image that we use to speed up builds. However, the steps mentioned there works only on GitLab CI, and not when someone is building a package in local machine. The workaround is to set COMPILE_ASSETS=true
. We need to update the docs to either only mention that, or give a detailed instruction on how to pull the asset image and place the contents in correct place.