Slight improvements to Dockerfile

I made two minor improvements to the Dockerfile.

My reasons for doing this was to make it easier to use ngnix as a base image. In my case to make a series of packaged web-apps.

The changes are:
1. Changed the location of nginx.conf so it is located in its own directory. Reason being if for some reason it would be modified when using this as base it might need to be shared with volumes.
2. Removal of source files after installation. This saves about 42MB currently, which is minor in itself. However if you do a series of web-app containers like me each instance matters to some.