Proxy configuration on Intel NUC image

I’m probably not following correctly but I don’t understand how. Here’s what I do:

$ sudo cp resin-nuctest-2.12.0+rev1-dev-v7.1.7.img /dev/sdb
$ sudo mount /dev/sdb2 /mnt
$ sudo mkdir /mnt/mnt/boot/system-proxy
$ sudo vim /mnt/mnt/boot/system-proxy/redsocks.conf

and I flash/install my NUC, it reboots and doesn’t work. I know my redsocks.conf is correct because on a development image if I get into the shell and copy the same file (from another USB stick) into /mnt/boot/system-proxy/redsocks.conf and reboot/restart redsocks service it works perfectly.