Dagger Build fn (CI side only)

[haydenk]

Parent

Part of #14

What to build

Add Build(ctx, source *dagger.Directory, distro, version string, platform dagger.Platform) *dagger.Container to .dagger/main.go. Use it from ci.yml's build job in place of the docker/build-push-action step (CI does not push).

Keep release.yml on docker/build-push-action — auth, manifest list, and OCI label semantics on the push path are best handled there. The migration of the push path is explicitly out of scope.

Acceptance criteria

• dagger call build --source=. --distro=debian --version=bookworm-slim --platform=linux/amd64 succeeds locally
• Multi-platform (linux/amd64 + linux/arm64) build verified in CI via dagger
• ci.yml build job uses dagger
• release.yml release job unchanged
• GHA cache backend continues to populate
• PAT-less

Blocked by

None — can start immediately.