Add install options for docker and Linode marketplace

[hughbris]

Thought these needed to be covered. Miscellaneous tweaks to go along.

Hopefully you don't see linking to my own Caddy docker image as self-promotion, it just happens to be the only working one I know of. I made it for myself and don't care if anyone else uses it.

[hughbris]

Added another commit when I realised marketplace app should also be added to Linode instructions.

[rhukster]

Thanks!