envoy: cross compile instead of compiling under slow emulation - #3152

Part of agent-substrate#3217. The default atenet dataplane runs envoy-dataplane in the
router and egress gateways, but releases do not publish it, so a
pre-built install has to use `--atenet-dataplane=agentgateway` or build
from source. agent-substrate#2112 dropped it because building Rust under QEMU was slow.
agent-substrate#3152 made the Dockerfile cross-compile, so the release job now builds
it with buildx and no QEMU.

Released Dockerfile images are an allowlist (`releasedDockerfiles`) next
to the ko allowlist. The release jobs pin BuildKit by digest. Release
builds take each Dockerfile image's digest from buildx's
`--metadata-file`, build Dockerfile images before the ko images, ignore
`DOCKER_BUILD_FLAGS`, and label every image with its source, revision
and version. With every image a default install deploys now published,
the release notes give the install command.

After the first release that includes it, an org owner has to make the
new `ghcr.io/agent-substrate/substrate/envoy-dataplane` package public.

### Validation

`release.yaml` from this branch, run in my fork at 619f0d9 with a
throwaway tag `v0.0.0-envoytest.1`:

-
[`images-dry-run`](https://github.com/MushuEE/substrate/actions/runs/40562367990),
dispatched with `dry_run_images`, built all eleven release images cold
on `ubuntu-latest` and passed the platform and digest checks in 11m35s,
well inside the 30-minute timeout.
- The [tag push
run](https://github.com/MushuEE/substrate/actions/runs/40561810523)
created the draft release. The images job skips itself outside
agent-substrate/substrate, as designed.

Earlier, at bbd8614, I ran the same publish locally against a throwaway
registry, as the `images-dry-run` job does:

```sh
docker run -d --rm --name ate-release-test -p 5000:5000 \
  registry:3@sha256:ddf754342cfc8acc51a56d5d0ab6af06826461864460636d8bd5c546dab2a7b8
printf '[registry."localhost:5000"]\n  http = true\n' > /tmp/buildkitd.toml
docker buildx create --name ate-release --driver docker-container \
  --driver-opt network=host --buildkitd-config /tmp/buildkitd.toml --use
VERSION=v0.0.0-local.1 KO_DEFAULTPLATFORMS=linux/amd64,linux/arm64 \
  go run ./cmd/ate-setup publish release-images --ko-docker-repo localhost:5000/substrate
docker buildx imagetools inspect localhost:5000/substrate/envoy-dataplane:v0.0.0-local.1
docker buildx imagetools inspect localhost:5000/substrate/envoy-dataplane:v0.0.0-local.1 \
  --format '{{json (index .Image "linux/amd64").Config.Labels}}'
```

- buildx cross-compiled the Rust module (`linux/amd64->arm64`), with no
QEMU.
- The envoy-dataplane digest from `--metadata-file` matched what the tag
resolved to.
- envoy-dataplane had `linux/amd64` and `linux/arm64`, and the `source`
and `revision` labels.
- A rerun built nothing.

That run showed envoy-dataplane inheriting
`org.opencontainers.image.version: "22.04"` from the Ubuntu base of
`envoyproxy/envoy`. Release images now set that label to the release
tag.

AI use: I used Claude Code to trace the code agent-substrate#2112 removed, draft the
change, address review comments, and script the local and fork
validation. I reviewed the diff and ran the ate-setup tests and the
validation above.

- [x] Tests pass
- [x] Docs updated

---------

Signed-off-by: Grant McCloskey <[email protected]>