Supporting whitelist configuration for # syntax = frontend would be useful for preventing malicious frontends from being specified due to e.g. typo.

The whitelist could be specified either via static buildkitd.toml or dynamic CRUD object.

whitelist types

• image reference prefix
• image digest
• notary signer key
  ...

dynamic CRUD object

For a while bbolt could be used, but in long-term, we might want to use distributed store like etcd for supporting multi-master distributed mode.