Running alternative Docker daemon on the same host

run-altdocker.sh

#! /bin/bash
# References:
# - https://stackoverflow.com/questions/32334167/is-it-possible-to-start-multiple-docker-daemons-on-the-same-machine
# - https://docs.docker.com/engine/reference/commandline/dockerd/

# First you need to have Docker 18.09+ and the bridge-utils package installed.
ROOT=/opt/altdocker

: ${bridge=altdocker0}
: ${base=$ROOT/$bridge}

if ! ip link show $bridge > /dev/null 2>&1
then
   brctl addbr $bridge
   ip addr add ${net:-"10.9.0.1/24"} dev $bridge
   ip link set dev $bridge up
fi

# NOTE: "--config-file" must be specified to avoid conflicts with system Docker.

dockerd \
  --exec-root=$ROOT/docker/run \
  --data-root=$ROOT/docker/data \
  --config-file=$ROOT/docker/etc/daemon.json \
  --storage-driver=overlay2 \
  --bridge=altdocker0 \
  --host=unix://$ROOT/docker.sock \
  --pidfile=$ROOT/docker.pid

# To use this Docker daemon,
# DOCKER_HOST=unix:///opt/altdocker/docker.sock docker run --rm -it alpine:3.8 sh -c "hello world"