Change datadir with docker?


I need to moove or create my data on my external hardrive ( local ) with docker.
without docker its easy.
We have the option datadir in pannel admin.
But with docker the option is no longer there …

With new install, i try : `

docker run -d -p 4443:4443 -p 443:443 -p 80:80 -v ncdata:/mnt --name nextcloudpi ownyourbits/nextcloudpi-armhf $IP

but not work … all data stay in sd card

I mount my hdd to “mnt” with : mount /dev/sda1 /mnt
and : chmod 775 mnt

Can you help me please ?
I m not dev, just an enthusiast of open source

-v /mnt/ncdata:/data

You can also check wiki