Turn on threads by default in Docker

Author: pavpanchekha

Dockerfile

@@ -22,4 +22,4 @@ ENTRYPOINT ["racket", "/src/herbie/main.rkt"]
 EXPOSE 80
 # NOTE --public allows the Docker host to interact with the demo,
 # typical users shouldn't need to use it.
-CMD ["web", "--public", "--port", "80", "--quiet", "--demo"]
+CMD ["web", "--public", "--port", "80", "--quiet", "--demo", "--threads", "2"]