ci: Add workflow to build disk image

[cgwalters]

And upload to ghcr.io because for some reason this isn't working with quay.io.

[cgwalters]

The image failure is unrelated, see #22

[cgwalters]

See an example successful run of this here: https://github.com/cgwalters/centos-boot-layered/actions/runs/7280872408/job/19840208917

[cgwalters]

To fetch the content, today this is technically possible with skopeo but has absolutely atrocious ergonomics:

$ skopeo copy docker://ghcr.io/cgwalters/fedora-bootc-cloud-disk:eln oci:disk
$ find disk/blobs/ -type f -size -5M | xargs -r rm
$ mv disk/blobs/* fedora-bootc-cloud-eln.qcow2.zst
$ rm -rf disk

After you'll want to decompress: zstd --rm -d fedora-bootc-cloud.qcow2.zst

Or...with oras:

$ oras pull ghcr.io/cgwalters/fedora-bootc-cloud-disk:eln

(And you'll still want to decompress)

Also in all the above we'll s/cgwalters/centos-bootc/ - though I need to figure out what's blocking shipping to quay.io here, I thought they supported general artifacts.

[sallyom]

LGTM
image build will pass when #22 merges

[cgwalters]

Ah OK this is failing because the GH workflow in this repository I think doesn't have permission to create packages - cc @lmilbaum

I need to dig into why quay.io is still allow-listing only certain media types which is blocking this use case of managing disk images there.

[lmilbaum]

@cgwalters GW workflow should be working now