Nothing showed where a published image or download came from, and the
standalone tarballs had no checksum at all. Anyone who could swap a
release asset could swap its `.sha256` with it.
- every published image (release, `-always`, beta/dev and Lite) gets a
signed build provenance attestation, pushed to GHCR next to it
- the Lite zips and standalone tarballs are attested as well
- standalone tarballs get a `.sha256`, like the Lite zips
- the tarball build no longer holds a write token: a separate release
job, which runs no checked-out code, checksums, attests and uploads
- the README says how to check with `gh attestation verify`
Adds a GitHub Action that builds the Next.js standalone output on
release and attaches architecture-specific tarballs (amd64 + arm64)
as release assets. Downstream projects can download and extract
instead of building from source.
Closes#178
Linus Rath
shuki