Complement QoL changes (#2663)

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)

.github/workflows/dendrite.yml

@@ -376,6 +376,8 @@ jobs:
       # Build initial Dendrite image
       - run: docker build -t complement-dendrite -f build/scripts/Complement${{ matrix.postgres }}.Dockerfile .
         working-directory: dendrite
+        env:
+          DOCKER_BUILDKIT: 1
 
       # Run Complement
       - run: |