[tormath1]

In this PR, we add a new step to the 'release' CI job. The goal is to publish release artifacts for alpha channel on Cloudflare Bucket to experiment with this kind of storage for release artifacts.

TODO:

• Setup r2.dev URL for public development URL (CNCF ticket is still open...)

Locally tested:

$ aws s3 ls s3://flatcar/ --recursive
2025-08-12 10:26:44       1237 alpha/amd64/4372.0.1/flatcar-container.tar.gz.DIGESTS.asc
2025-08-12 10:26:45        594 alpha/amd64/4372.0.1/flatcar-container.tar.gz.DIGESTS.sig
2025-08-12 10:26:45        318 alpha/amd64/4372.0.1/flatcar-container.tar.gz.DIGESTS
2025-08-12 10:26:44        594 alpha/amd64/4372.0.1/flatcar-container.tar.gz.sig
2025-08-12 10:34:12  409256107 alpha/amd64/4372.0.1/flatcar-container.tar.gz
2025-08-12 10:26:46       1216 alpha/amd64/4372.0.1/flatcar-incus.raw.DIGESTS.asc
2025-08-12 10:26:46        594 alpha/amd64/4372.0.1/flatcar-incus.raw.DIGESTS.sig
2025-08-12 10:26:46        297 alpha/amd64/4372.0.1/flatcar-incus.raw.DIGESTS
2025-08-12 10:26:47        594 alpha/amd64/4372.0.1/flatcar-incus.raw.sig
2025-08-12 10:38:02  122224640 alpha/amd64/4372.0.1/flatcar-incus.raw
...

This is the last step of the release job, so even if it fails, we can proceed with the release as usual.

Needs:

• docker: add rclone mantle#649