Offer Debian repository to keep Headscale installations up to date

Why

Currently the workflow of security checking of Headscale hosts is quite frustrating.

1. Regularly log into host and run dpkg -l | grep -i headscale to see which Headscale version is installed.
2. Go to the releases page of this Github repository.
3. Check most current release version of Headscale.
4. Download most current release version of Headscale.
5. Run sudo dpkg --install headscale.deb to update Headscale.

Description

Offer a package repository for Debian/Ubuntu which can be added to APT. That allows to either run apt update && apt upgrade or even use unattended-upgrades to update Headscale.