Support Docker Socket proxy

[robflate]

For security it would be nice to be able to use a Docker Socket proxy rather than adding the socket directly;

DOCKER_HOST: tcp://socket-proxy:2375

https://github.com/Tecnativa/docker-socket-proxy
https://docs.linuxserver.io/images/docker-socket-proxy/

[zix99]

Great callout. I did look at the library I use for docker integration, and it looks like it does automagically support that environment variable and the code is set up to use it. Would you be able to test that? If it works, all that's left is some documentation in the readme