feat: HA option for building harness config containers

[ptone]

Feature: HA option for building harness config containers

Context

The current scion build / harness config container build feature relies on a local Docker daemon. This works in single-node deployments but is incompatible with HA (multi-replica hub) deployments where:

• Replicas may not have access to a local Docker daemon
• Build artifacts from one replica are not available to others
• A centralized, repository-backed build is needed for consistency

Proposed Solution for HA Mode

1. Cloud Build integration: When running in HA mode, delegate container builds to GCP Cloud Build rather than local Docker. This avoids requiring a Docker daemon on hub replicas.
2. Push to configured repository: After a successful build, the resulting image must be pushed to the configured container registry (e.g. gcr.io/<project>/<image> or ghcr.io/<org>/<image>) so all hub replicas can pull it.

Behavior Split

Mode	Build mechanism	Image availability
Single-node / local	Existing local Docker build (unchanged)	Local daemon only
HA mode	GCP Cloud Build	Pushed to configured registry, available to all replicas

Requirements to Consider

• Auth: Cloud Build requires a service account with Cloud Build Editor + Artifact Registry Writer (or equivalent)
• Registry config: The target registry/repo must already be configured in hub settings
• Status feedback: Cloud Build is async — hub needs to poll build status and surface progress/errors to the user
• Cost/quota: Cloud Build has per-minute pricing; this should be documented in the feature