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)

test/http.go

@@ -68,7 +68,7 @@ func ListenAndServe(t *testing.T, router http.Handler, withTLS bool) (apiURL str
 		if withTLS {
 			certFile := filepath.Join(t.TempDir(), "dendrite.cert")
 			keyFile := filepath.Join(t.TempDir(), "dendrite.key")
-			err = NewTLSKey(keyFile, certFile)
+			err = NewTLSKey(keyFile, certFile, 1024)
 			if err != nil {
 				t.Errorf("failed to make TLS key: %s", err)
 				return