`generate_signing_key: not found` error in `Synapse.Dockerfile` for any synapse older than 1.54

[ShadowJonathan]

Via #328 and matrix-org/synapse#12118

(1.54 is technically not out yet, but the latter PR puts it on develop for that release)

[DMRobertson]

The underlying root cause is #330.

[DMRobertson]

Going to close this as done: the config to make Synapse interplay with complement is now versioned within Synapse (#330/#359).