Enforce Complement test Docker build times in CI

[matrixbot]

This issue has been migrated from #14316.

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As mentioned in the weekly backend team call and in the internal backend room

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Testing with Complement has gotten so slow, matrix-org/synapse#13204

Before people put effort into making it faster again, it would be nice to add some CI to enforce that we don't regress the timing. We've fixed and regressed the timing many times so I think it's worth actually enforcing. And as @clokep describes it, "slow tooling really impacts velocity."

What we want to test is that a rebuild after a Synapse source change is fast. Not concerned with initial build times here since that's not the primary dev experience when iterating on a feature/bug.

Potential solutions

Add a Complement Docker image rebuild job and fail the CI if it goes over a threshold duration.

It would be nice to do this as part of the existing Complement job since it has to do the initial Docker build there anyway.