Ok I know its not free, but I use these for all my hosting needs, albeit in the form of a virtual server.
It costs £3 a month Inc VAT and you get full root access, and there is Debian on there which is very much like the raspberry pi so the install guide is near enough like for like.
I am not affiliated with them, just passing on my good findings from when I was looking for a cheep VPS to run my website and experiment with mail servers.
Their support is slow, but I’ve only needed it once when one of the disc was on its way out. Rest of the time, its a linux box, so “just works”.
Hope that helps