remove error messages, enable rapid shutdown

[jbarratt]

When using your (very handy) container, I noticed

• it was spitting out some error messages when starting up, as it was attempting to run 443 and 80 as commands
• it was taking 10 seconds to docker stop the container, because the wrapper shell script (was wrapping another shell script that was wrapping another shell script) so the SIGTERM sent by docker stop was not getting to nginx.

This PR addresses both of those issues.

[jbarratt]

Using the square brackets form means it doesn't run sh -c "sh configure_nginx.sh", it just runs sh configure_nginx.sh.

[cbachich]

Thanks Joshua! These are some good improvements. I'll merge them. I'm also going to take this opportunity to add automated builds for this project. I'll update the README and you can check it at: https://registry.hub.docker.com/u/cbachich/docker-ssl-proxy/

[jbarratt]

That's fantastic, thanks!