Request a custom template build (queued for the isolated builder)

202 with the queued build. Takes recipe v1 (a Dockerfile) or recipe v2 (: validated against the base, compiled to the builder’s steps, uploads locked). Records the canonical recipe and its SHA-256 (provenance input), pins the base version, clamps resources to the plan and assigns target_version. The cell executes it (building -> testing -> publishing -> published) and the API then registers the version (published at once unless auto_publish=false); poll GET .../template-builds/{id} until registration.state is registered (or the build failed/was canceled). builder_availability says whether a builder is running. Errors: 422 when the recipe is outside the host builder Dockerfile dialect or the slug is not a builder slug (details.reason: multi_stage_not_supported, from_not_template_base, stage_names_not_supported, from_flags_not_supported, base_mismatch, run_flags_not_supported, heredoc_not_supported, instruction_not_supported, no_build_context, env_invalid, user_invalid, too_many_steps, recipe_too_large, slug_not_supported_by_builder, platform_template_slug, reserved_template_slug, base_not_found, base_not_published, base_archived, architecture_not_supported, ...; line for line errors; recipe v2: invalid_recipe (details.field, details.detail), base_not_layered, language_unavailable, language_conflict, invalid_package, too_many_files, platform_owned_path, invalid_path, upload_required, upload_missing, upload_digest_mismatch, upload_too_large, too_many_steps, recipe_too_large, allowlist_empty, allow_hosts_without_allowlist, extra_hosts_without_auto, too_many_hosts, invalid_host, ip_literal_not_allowed, host_not_allowed, invalid_settings, services_unsupported, too_many_acknowledged_findings; immutable paths: invalid_path (details.field recipe.immutable[<i>]), immutable_path_removed (the recipe drops a path of the template’s open version; details.removed), immutable_paths_unsupported_base (a non-empty list on a base whose guest agent lacks immutable_paths.v1, or a recipe v1 build of a template with immutable paths; details.base {name, version})), 503 dependency_unavailable (uploads_not_configured), 402 entitlement_required, 429 rate_limited (template_build_queue_full: already 10 builds queued past the plan’s concurrent_template_builds; retryable, Retry-After; a build past the plan’s slots is accepted and waits, its queue gives the position and expected wait), 409 template_archived. Supports Idempotency-Key.