Cannot access to mounted volume

Hello,
I fail to access to my mounted volume /prometheus/

docker run -d --volume ‘/opt/monitor-server/conf:/prometheus’ prom/alertmanager:v0.16.2

docker exec -it 630cfbf66fc1 /bin/sh

I have a error message:

/ $ ls
alertmanager bin dev etc home lib proc prometheus root sys tmp usr var
/ $ cd prometheus/
/bin/sh: cd: can’t cd to prometheus/: Permission denied

Does somebody could help me ?

Regards.
Frédéric

what is the rights of this folder: /opt/monitor-server/conf ? what about the files in it?