Fix watch-serve.sh for docker

[tsuzu]

The error below is printed when I executed watch-serve.sh with docker.

❯ ./watch-serve.sh
Using '/usr/local/bin/docker' as container runtime
"docker build" requires exactly 1 argument.
See 'docker build --help'.

Usage:  docker build [OPTIONS] PATH | URL | -

Build an image from a Dockerfile

docker build will accept Dockerfile from stdin if - is passed