Design architecture for trusted third parties to easily add worker nodes

[theory]

Iterate on the design output from #63 to design an architecture for trusted third parties to easily add worker nodes. The Postgres build farm, including its source code might provide some precedent. But carefully consider how to:

• Ensure the integrity of such systems remain unviolated
• Only trusted systems are allowed to submit results to publish
• Provide an interface for managing those nodes
• How to avoid duplicate builds while making things resilient, potentially by relying on multiple nodes for a given OS/architecture
• How to remove images built by a node later found to be compromised