preview.sh: Allow preview.sh keep running for multiple build.sh calls
    
    antora deletes and recreates public/ every time the docs are rebuilt;
    mounting public/ directly meant that when build.sh, the existing nginx
    container didn't see the new docs. Instead of mounting public/ directly
    into the nginx default webroot, mount the parent directory at /antora,
    and install a tiny nginx configuration file to point the webroot at the
    right place.