That command is supposed to be in a hook, not to be executed directly in website.git
$ cat > hooks/post-receive
#!/bin/sh
GIT_WORK_TREE=/var/www/www.example.org git checkout -f
$ chmod +x hooks/post-receive
That means you need to create a website.git/hooks/post-receive file, with that command in it, and make it executable.
That will allow you to push commits top that bare repo (website.git), which will trigger the post-receive hook, and will checkout the repo content into a different working tree.
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