Implement auto-watching of certfiles and config files, and multiple explicitly configured watch files

Currently the image requires a $WATCH_FILE to be explicitly set. We should instead be automagically watching:

• logfiles this is way more complicated, moved to #2
• certificate/key files
• config files
• multiple explicitly configured paths (via WATCH_PATHS envvar)

Once any change happens to any watched path, we kill -USR the nginx process.