Build assets only image

fetch_assets is failing to download assets from tagged release v11.4.0-rc2-ee as well as v11.4.0-rc1-ee. It does work for CE tags v11.4.0-rc1 and v11.4.0-rc2

Pipeline for the tag is https://gitlab.com/gitlab-org/gitlab-ee/pipelines/32336696

Downloading assets manually through web works

This is preventing CNG images for the RC from being built

Switch to building a docker image containing only the assets