Hello ilias,
yes, it runs an Resin OS 2.0.0+rev3 on an raspberry pi zero (my resin server)
Mounting external storage is for sd-cards and usb sticks, not for nfs servers?
I can mount the nfs volume, when I call the terminal inside the dashboard.resin.io. After that I can e.g. browse the directory from the running docker app. ( I run sabnzbdplus as a docker app and browse during web interface)
So, you cannot mount the nfs volume at build time (i.e. in your dockerfile). But if you can mount the nfs volume when you open the web terminal (from your resin.io dashboard) then you should be able to run the same command in a script at boot time. So you can create a script and put a RUN command at the end of your dockerfile.