This makes sense, because docker is only listening on localhost.
Now, I think your nextcloud is making requests to the port 9980 which it should not do. Instead it should use the apache proxying which you configures. As admin in nextcloud, go to
/settings/admin/richdocuments and make sure that the URL you give there is something like
http://collabora.ertuit.fr/ and nothing with 9980.
If this is not the problem, I am out of ideas. As I said, the tutorial assumes that you do everything with real hostnames. Moreover, plain http instead of https might be a problem.