Post-install fails (on minimal Debian Buster) when gpio-group is missing

[gitdings]

Using the current image from https://raspi.debian.net/, install of argon1_0.2.1_all.deb fails:

Setting /usr/bin/argonone-shutdown to suid argonone Adding user argonone to gpio and i2c groups usermod: group 'gpio' does not exist dpkg: error processing package argon1 (--install): installed argon1 package post-installation script subprocess returned error exit status 6

After adding gpio goup (i2c was there), post-install is happy.

P.S. Putting this in a Debian package is the right thing to do. Thank you!