Build an assets only container

What does this MR do?

Store the built assets in a public container for use in the build process for CNG and omnibus-gitlab

What are the relevant issue numbers?

https://gitlab.com/charts/gitlab/issues/816

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides