Update entrypoint.sh to require exactly one configuration file

[jordanpotter]

Improvements from the conversation in issue #48, which identified that the image doesn't correctly handle when multiple configuration files are provided.

This change ensures the container errors out explicitly if more than one configuration file is detected.

[BobSammers]

This looks good to me. A quick test proves it does what it promises and, as you say, it makes config choice explicit and straightforward.

My only note is that filenames containing whitespace cause issues, but a little experimentation suggests actually trying to use such names opens a whole can of worms with wg-quick and invalid interface names, so I don't think it's worth spending any time on. Only a masochist would try this anyway.