read-only ftp user possible or not?

[joximu]

You're right:
all ftp users of one customer (domain) have the same uid and gid.

So, either you change the uid of the to-be-read-only-user to another value (something like 1500 - a non existent value) but leave the gid as is - then change the permissions so that groups only can read (I did not test this).
Or make it via web/http -> a folder with htuser protection.

/J