How to install a certificate in trusted

I am trying to install a certificate on my devices, but I must be missing a step because it keeps saying 0 added. Any assistance would be greatly appreciated.

mkdir /usr/share/ca-certificates/extras
cp /tmp/fortigate.crt /usr/share/ca-certificates/extras/fortigate.crt
dpkg-reconfigure -p critical ca-certificates
0 added, 0 removed; done.

This is FROM resin/raspberrypi3-debian:stretch

I successsfully installed this using the same procedure on another debian machine, although that one was jessie, I am not aware of any differences that exist with the procedure.