You may try this instead:
<IfModule mod_rewrite.c>
Options +FollowSymlinks -MultiViews
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} !/app/webroot/shop/? [NC]
RewriteRule ^shop/(.*) /app/webroot/shop/$1 [L,NC]
RewriteCond %{REQUEST_URI} !/app/webroot/? [NC]
RewriteRule ^$ /app/webroot/ [L,NC]
RewriteCond %{REQUEST_URI} !/app/webroot/? [NC]
RewriteRule ^(.*) /app/webroot/$1 [L,NC]
</IfModule>