Run entrypoint script with ENTRYPOINT instead of CMD

[muripic]

Fixes #65.

As stated in the official doc, the main purpose of the CMD instruction is to provide a default. If this default is overridden when running docker run, the default will not run.
A better instruction to run the entrypoint script, therefore, is ENTRYPOINT, which will always run unless it's explicitly overridden with the --entrypoint flag.

[muripic]

I testedI got an error when using it with docker-compose because the entrypoint script actually executes ngrok, and realized that these use cases are covered by passing a value such as service:8080 to the envvar NGROK_PORT.