Balancker: poor man's load-balancer for docker

Balancker picks up random $DOCKER_HOST from ~/.config/balancker

Config (~/.config/balancker):

# NOTE: ssh:// requires Docker v18.09 or later
ssh://node1
ssh://node2
# If you are using Docker before v18.09, forward the socket manually with
# `ssh -L` (or set up TLS)
unix:///path/to/docker.sock

Random mode:

$ eval $(balancker)
$ docker build -t foo /foo

Deterministic mode with seed argument:

$ eval $(balancker bar-seed)
$ docker build -t bar /bar