soci-snapshotter: render registry mirrors as resolver hosts

[komapa]

Issue number:

Closes bottlerocket-os/bottlerocket#4949

Description of changes:

packages/soci-snapshotter/soci-config-toml rendered settings.container-registry.mirrors as:

[plugins."io.containerd.grpc.v1.cri".registry.mirrors."<registry>"]
endpoint = [...]

Only the SOCI containerd plugin reads that section (service/plugin/plugin.go). Bottlerocket runs the standalone soci-snapshotter-grpc daemon, which reads mirrors only from [resolver.host."<registry>".mirrors] and ignores unknown keys, so configured mirrors were never used by SOCI. With snapshotter = "soci", containerd resolves manifests through the mirror, but SOCI fetches all layers from the upstream registry.

This change renders each endpoint as a resolver host mirror:

[[resolver.host."<registry>".mirrors]]
host = "<endpoint>"

and drops the now-unused join_array helper from the template requirements.

Compatibility: with SOCI 0.16, parallel pull still does not fetch layers from mirrors, and "*" and http:// mirror hosts are not handled; companion PRs awslabs/soci-snapshotter#2095 (mirrors in parallel pull, "*", URL schemes) and awslabs/soci-snapshotter#2096 (distribution source labels) address that. Until SOCI is updated, the rendered mirrors have no effect on pulls, which matches today's behavior.

Testing done:

• Rendered the template with schnauzer (render_template_str + FakeImporter) for mirrors "*" and an ECR registry with two endpoints; output:

  [[resolver.host."*".mirrors]]
  host = "http://10.0.0.1:30031"
  [[resolver.host."123456789012.dkr.ecr.us-east-1.amazonaws.com".mirrors]]
  host = "http://10.0.0.1:30031"
  [[resolver.host."123456789012.dkr.ecr.us-east-1.amazonaws.com".mirrors]]
  host = "https://mirror.example.com"

• Loaded that file with SOCI's config.NewConfigFromToml on the companion SOCI branch: an ECR image resolves to http://10.0.0.1:30031, https://mirror.example.com, then the ECR endpoint.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.