many: copy to /root and add rebuild instructions

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This fixes COPY verbs for /root and adds instructions to each containefile.

This also adds a default ARCH value of amd64 so x86_64 users can rebuild without any podman ENV arguments from the get-go.

I tested this on qcow2 locally, then used Cursor to apply the same updates to other files.

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[2026-05-07T11:58:56,357953300+00:00] Setup prefetched
Trying to pull quay.io/hummingbird-ci/bootc-os:latest...
Error: unable to copy from source docker://quay.io/hummingbird-ci/bootc-os:latest: initializing source docker://quay.io/hummingbird-ci/bootc-os:latest: reading manifest latest in quay.io/hummingbird-ci/bootc-os: unauthorized: access to the requested resource is not authorized

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/retest

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Rebased on top of #84 maybe CICD will work?!?

[lzap]

All good.