[Feature request] Custom docker socket

[Klubas]

I'm using BalenaOS, and it uses balena-engine instead of docker.

If I enable Docker integration i get the error "[ERROR] Can't connect to the Docker API on localhost", that could be solved changing the docker socket to "unix:///var/run/balena-engine.sock" here
https://github.com/pawelmalak/flame/blob/master/controllers/apps/docker/useDocker.js

I >think< it could be implemented using an environment variable. If there's is interest I can make a pull request with this option.

[el-joseppe]

this would be also necessary for anyone (like myself) who wants to limit the docker-socket-access via a docker-socket-proxy! So I would appreciate the addition to configure the socket via environment variable!

[Klubas]

Finally got around to making a PR for this. #396