Revert "containers: Move core requirements into base image"

This reverts commit 120a086a.

It turns out that we were not automatically building and deploying the
base image on the CI, like we are for the ci image. And it's hard to do
that for various reasons. So let's just revert this not-so-great tidy-up
and live with having to explicitly manage all the containers'
Dockerfiles.

Fixes: 120a086a ("containers: Move core requirements into base image")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>