add docker install hints

[charlesfrye]

Installing docker can be painful -- the recommended process involves manually doing GPG key stuff, iirc.

On Ubuntu, IIRC snap makes installation of docker easier. I should look into that and see what the drawbacks are.

[charlesfrye]

snap can make things harder, it turns out. most commands in docker tutorials/docs assume the dockerd runs differently than it does in snap. installing docker with snap and then trying to fix it kinda broke my system