fix(build): pass the shared buildx flags from every image target - #4485

## What this PR does

Package images can now be built for amd64 and arm64 in one `make image`,
and a published build is multi-arch unless `PLATFORM` says otherwise.
This is the build half of arm64 support from #1961 and #903. CI still
publishes amd64 only until it gets a native arm64 leg, which comes in a
separate PR.

Builder stages now run on the build platform and compile for the target
one, so Go never runs under emulation. Under emulation it is slow and it
crashes: the migration-controller Dockerfile already records a "bad
pointer in Go heap" from exactly that. On top of this, `PLATFORM`
defaults to `linux/amd64,linux/arm64`. A build on an arm64 workstation
can't publish an arm64-only digest any more. That happened once already,
the cilium pin was arm64-only until 3f36a1b rebuilt it by hand.

Two images were only right because they were emulated. On a native
builder they would ship the wrong architecture. token-proxy ran `go
build` with no GOARCH, and kube-ovn calls upstream `make build-go`,
which hardcodes `GOARCH=amd64`. Both are fixed now. The new bats test
wants every compiling stage on `$BUILDPLATFORM` and actually using
`TARGETARCH`. A bare `ARG TARGETARCH` does not count, this is how
token-proxy got through.

Some things stay amd64. talos and testing pin it with `override`,
because matchbox serves amd64 Talos assets and the sandbox runs
`qemu-system-x86_64`. `LOAD=1` builds for the host only, since the
classic docker image store can't load an index. Every CI build job pins
`PLATFORM: linux/amd64`, so CI output does not change.

This changes local builds. A `make image` on the default docker driver
with the classic image store now fails with "Multi-platform build is not
supported for the docker driver", where before it built for the host.
Use a `docker-container` builder (`BUILDER=<name>`), or pass
`PLATFORM=linux/amd64` (or `LOAD=1`) to build one architecture.

I checked it on an arm64 host with a docker-container builder.
cozystack-api, dashboard, kubeovn, platform-migrations, cilium and
capi-providers-cpprovider build and push as amd64+arm64 indexes.
Binaries pulled from each platform are x86-64 and aarch64:
cozystack-api, token-proxy, the kamaji provider manager, cilium-agent
and all four kube-ovn binaries. piraeus-server builds for both
platforms, but the push to my test registry failed on registry 500s. The
capi-providers-cpprovider inline cache works with a multi-platform push.
Not covered is running the arm64 images on an arm64 cluster, CI has
none.

This is stacked on #4485 and targets its branch, so the diff here is
only the two commits on top.

### Screenshots

No UI changes.

### Downstream repositories

- [ ] No downstream repository is affected by this change
- [x] [cozystack/website](https://github.com/cozystack/website) -
follow-up: cozystack/website#711
- [ ]
[cozystack/terraform-provider-cozystack](https://github.com/cozystack/terraform-provider-cozystack)
- follow-up:
- [ ]
[cozystack/ansible-cozystack](https://github.com/cozystack/ansible-cozystack)
- follow-up:
- [ ] [cozystack/ccp](https://github.com/cozystack/ccp) - follow-up:
- [ ] [cozystack/talm](https://github.com/cozystack/talm) - follow-up:
- [ ] [cozystack/cozyhr](https://github.com/cozystack/cozyhr) -
follow-up:
- [ ] [cozystack/cozy-proxy](https://github.com/cozystack/cozy-proxy) -
follow-up:
- [ ]
[cozystack/cozystack-telemetry-server](https://github.com/cozystack/cozystack-telemetry-server)
- follow-up:
- [ ]
[cozystack/external-apps-example](https://github.com/cozystack/external-apps-example)
- follow-up:
- [ ] [cozystack/examples](https://github.com/cozystack/examples) -
follow-up:
- [ ] [cozystack/community](https://github.com/cozystack/community) -
follow-up:

### Release note

```release-note
feat(build): package images build for linux/amd64 and linux/arm64 by default, with Go builders cross-compiling on the build platform; a local `make image` now needs a docker-container builder, or PLATFORM=linux/amd64 / LOAD=1 for a single architecture. CI still publishes amd64.
```


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Build Improvements**
* Container image builds now target both amd64 and arm64 by default when
building without loading images locally.
* CI and release builds are pinned to amd64; Talos and testing images
remain amd64-only.
* Cross-platform builds now compile on the builder’s native platform
while targeting the requested architecture.
* **Documentation**
* Clarified platform defaults and architecture requirements for local
and CI builds.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

…leases on it (#4505)

## What this PR does

Pre-release builds now publish every image as an amd64 and arm64 index,
and a release fails if anything it ships is single-arch. Main, release
lines and pull requests keep building amd64 only. It builds on #4498,
which made every image buildable for both architectures. Part of #1961.

**This PR is on hold until everything under "Merge after" has landed.**
Merged earlier, the new gate fails the next rc on the images those PRs
fix.

### How a pre-release is built

A pre-release tag gets a second job, for arm64, next to the existing
release job. That covers `-rc.N`, `-beta.N` and `-alpha.N` tags. It runs
on the CNCF arm64 pool (`oracle-vm-24cpu-96gb-arm64`), builds the same
images with `PLATFORM=linux/arm64`, and pushes them as `<tag>-arm64`.
`prepare-release` waits for it, and if the arm64 job fails the rc fails
with it. There is no amd64-only fallback. Stable tags do not run it:
promotion copies the rc digests with `skopeo copy --multi-arch all`, so
a stable release inherits the indexes.

After the amd64 build, the stitch joins each amd64 image with its arm64
twin into one index. It lives in `hack/stitch-multiarch.sh`. Every tag
of the amd64 image moves to the index, the component-versioned ones
included. The script then rewrites the digests in the tree and
republishes the packages artifact and the installer chart pinned on
them. It reads refs through `hack/lib/image-refs.sh`, and it fails if an
old digest survives anywhere outside `charts/`.

### The gate

A new check fails the rc when a pinned digest in the tree is not an
index with both linux/amd64 and linux/arm64. It lives in
`hack/verify-multiarch.sh`. It checks first-party and third-party refs
alike. Images that are amd64 by nature go into
`hack/multiarch-allowlist`, one repository per line, each with a
mandatory reason. Today that is only the e2e sandbox. An entry that
matches nothing is reported as stale.

The static gate only sees digest-pinned refs in the tree. A tag-only
ref, an image from a vendored chart default, or one an operator starts
at runtime is invisible to it. So the rc e2e also audits every image its
nodes pulled, with the same check and the same allowlist. Neither covers
a package the e2e never installs.

### Nightly

A new nightly workflow builds main for arm64. It keeps the arm64 build
cache warm, since the rc job only reads it. It also prints the
single-arch refs that would block the next rc. Nothing it builds is
published.

### Smaller changes

- `CACHE_TAG` in `hack/common-envs.mk` moves the default cache ref, so
the arm64 build keeps a cache of its own.
- `MATRIX_ARCH=arm64` makes `hack/build-matrix.sh` leave out the
amd64-only e2e sandbox.
- The kamaji provider image is pushed under the build's `IMAGE_TAG` like
every other image. Its Makefile set `IMAGE_TAG` itself, so concurrent
builds overwrote each other's component tag. Closes #4503.
- `hack/nightly-mirror.sh` verifies a copy against the raw manifest
digest. Plain `skopeo inspect` resolves an index to one platform's
child.
- matchbox is rebuilt for both platforms in the amd64 release job. The
arm64 leg skips the talos package, so there is no arm64 half to stitch,
and its Dockerfile only copies files, so no QEMU is needed. Releases
then network-boot arm64 machines too. Closes #4524.

The stitch also rewrites the kamaji ref inside `files/components.gz`,
recompressed with `gzip -n` so the bytes are reproducible, so the kamaji
control-plane provider ships multi-arch too. keda and kuberture now name
the repository next to their pinned digest, so the gate can resolve
them; their rendered manifests do not change.

### Merge after

- #4552 builds the Talos installer, matchbox, Harbor and the Velero
KubeVirt plugin for arm64, and adds flux-plunger, keycloak-operator,
kilo and migration-controller to the root `build:` list. It replaces
#4507, #4515, #4525 and #4528.
- #4549 moves ingress-nginx, cozy-proxy, cozystack-scheduler and
keycloak-kms-proxy to their multi-arch releases, which are already
published.

The rest of the stack is merged: #4485, #4498, #4512, #4517, #4521 and
#4522 here, and the multi-arch PRs in
ingress-nginx-with-protobuf-exporter, cozy-proxy, cozystack-scheduler
and keycloak-kms-proxy.

### Verification

The bats suites pass: stitch, verify-multiarch, build-matrix,
common-envs, nightly-mirror and the release contracts. The first three
also pass under `hack/cozytest.sh` with dash. Each new test was red
before its implementation, and the verify-multiarch and stitch tests
each have a mutation that turns them red. I ran `verify-multiarch
--report` against main. It lists the first-party refs the stitch will
fix, plus the third-party and special cases above. The workflows have
not run yet. The arm64 job's tools and duration, the stitch against real
registries, and the audit are checked for the first time on the nightly
and on the next rc.

### Screenshots

Not a UI change.

### Downstream repositories

- [x] No downstream repository is affected by this change
- [ ] [cozystack/website](https://github.com/cozystack/website) -
follow-up:
- [ ]
[cozystack/terraform-provider-cozystack](https://github.com/cozystack/terraform-provider-cozystack)
- follow-up:
- [ ]
[cozystack/ansible-cozystack](https://github.com/cozystack/ansible-cozystack)
- follow-up:
- [ ] [cozystack/ccp](https://github.com/cozystack/ccp) - follow-up:
- [ ] [cozystack/talm](https://github.com/cozystack/talm) - follow-up:
- [ ] [cozystack/cozyhr](https://github.com/cozystack/cozyhr) -
follow-up:
- [ ] [cozystack/cozy-proxy](https://github.com/cozystack/cozy-proxy) -
follow-up:
- [ ]
[cozystack/cozystack-telemetry-server](https://github.com/cozystack/cozystack-telemetry-server)
- follow-up:
- [ ]
[cozystack/external-apps-example](https://github.com/cozystack/external-apps-example)
- follow-up:
- [ ] [cozystack/examples](https://github.com/cozystack/examples) -
follow-up:
- [ ] [cozystack/community](https://github.com/cozystack/community) -
follow-up:

Nothing under `hack/` is moved or renamed, and no make target changes
its default behaviour: `CACHE_TAG` and `MATRIX_ARCH` are opt-in. The
satellite repositories listed under "Merge after" are prerequisites, not
follow-ups this change forces on them.

### Release note

```release-note
ci(release): pre-release builds publish every image as an amd64 and arm64 multi-arch index, and a release fails if any image it ships or pulls in e2e lacks either architecture. Stable releases inherit the indexes through promotion.
```


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Release candidates now build amd64 and arm64 images in parallel and
combine eligible images into verified multi-architecture indexes before
publication.
* A nightly arm64 image build is available for testing and supplies a
build cache for release-candidate builds.
* **Bug Fixes**
* End-to-end checks audit whether pulled images support both
architectures. Audit failures block prerelease checks, while stable
releases continue with a warning.
* End-to-end test artifacts now include collected image references and
multi-architecture audit results.
* **Documentation**
* Updated release and image guidance covers multi-architecture builds,
verification checks, and troubleshooting, including arm64 build and
stitching failures.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->