base-imagectl: support --cachedir; use in Containerfile

Dusty Maberequested to merge
dustymabe/fedora-bootc-base-images:dusty-cache into main

As far as I can tell --cachedir was used prior to bootc-base-imagectl being introduced in c89b6f42. Let's take advantage of it again so that local build iterations can be quick.

Note also the second commit where an issue is worked around.

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