/etc/os-release supports a tag ID_LIKE which should be set to debian, see os-release(5):
diff -u -r1.1 /etc/os-release --- /etc/os-release 2018-04-27 19:27:26+02 1.1 +++ /etc/os-release 2018-11-29 21:40:06+01 @@ -1,6 +1,7 @@ PRETTY_NAME="PureOS" NAME="PureOS" ID=pureos +ID_LIKE=debian HOME_URL="https://pureos.net/" SUPPORT_URL="https://puri.sm/faq/#faq-pureosandsoftware" BUG_REPORT_URL="https://tracker.pureos.net/"