gid of `docker.sock` in host will be changed as it will be overwritten by container's docker

[mookjp]

Reporting in this PR: #150

My host has docker group and docker.sock owned by root:docker, 0660.
Starter script chown root:docker /var/run/docker.sock against mounted docker.sock, that changes owner gid of host's docker.sock.
To avoid this, I change container's docker gid to host's one with

${POOL_BIN_PATH}/docker-enter pool groupmod -g $(grep docker: /etc/group | awk -F ':' '{ print $3 }') > docker