support appending volumes

[briceburg]

It would be nice to devise a label strategy that allows appending values, for instance to add additional volumes. Currently you need to lookup the volumes label from upstream Dockerfile(s), and then copy this value while appending your changes.

E.g. the briceburg/ansible:2.1-dex image provides the label org.dockerland.dex.docker_volumes="~/.ssh/id_rsa:/dex/home/.ssh/id_rsa:ro \$ANSIBLE_CFG:\$ANSIBLE_CFG:ro ~/.ansible.cfg:/dex/home/.ansible.cfg:ro /etc/ansible/ansible.cfg"

If we want to add an additional volume in a downstream image, we have to copy this label and add the volume (say /nfs) -- org.dockerland.dex.docker_volumes="/nfs:/nfs:ro ~/.ssh/id_rsa:/dex/home/.ssh/id_rsa:ro \$ANSIBLE_CFG:\$ANSIBLE_CFG:ro ~/.ansible.cfg:/dex/home/.ansible.cfg:ro /etc/ansible/ansible.cfg" which becomes cumbersome and fragile.