This PR does the following:
- adds a `keysize` parameter to `generate-keys`, so we can use lower sized keys when running in CI
- updates the Complement docker files to use BuildKit (requires Docker >18.09)
- uses `exec` when executing `dendrite-monotlith-server`, making it PID 1 inside docker, which results in Dendrite actually receiving the `SIGTERM` signal send by Docker. (Making it faster when running tests with Complement, as we don't take 10 seconds to timeout)
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