Proxmox 4 is based on Debian Jessie. Reference for this install: https://docs.docker.com/engine/installation/linux/debian/#/install-using-the-repository
$ apt-get install curl
$ curl -fsSL https://apt.dockerproject.org/gpg | apt-key add -
$ echo "deb https://apt.dockerproject.org/repo/ debian-jessie main" > /etc/apt/sources.list.d/docker.list
$ apt-get update