Unable to bind directories

I am trying to bind a directory inside a Docker container and this is the result that I get:

[root@playground/]# mount --bind /etc/ /mnt
mount: /etc is write-protected, mounting read-only
mount: cannot mount /etc read-only

/etc/ is not write-protected.
What am I doing wrong?

I’ve already tried to use --cap-add=SYS_ADMIN.