This is mostly done to remove the hardcoded HOME
env var that ci-templates appends to the image. See [1]
[1] https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/2433#note_2243222
But it also allows us to add extra things useful for local
development to it, as long they don't interfere with the
build enviornment.
This also means we can switch the build image to be based
on top of the normal fedora one rather than toolbox,
so it will no longer advertise the toolbox compatible labels
since it was buggy anyway.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7455>
