Slaves with no persistent_disk cannot run docker

[mrdavidlaing]

Docker support is added by co-locating the docker-boshrelease.

The docker-boshrelease hardcodes docker to store its "graph" in /var/vcap/store, meaning that on a node with no persistent_disk this is the 2GB root volume.

This leads to errors for builds that use docker like this:

2014/08/15 10:28:15 Error pulling image (latest) from pandrew/awscli, write /var/vcap/store/docker/docker/aufs/diff/fc9a6a985bf3dea9b50cbea54316d259fd28933be16565e6a96e8d4379d400b7/usr/lib/perl/5.14.2/auto/Encode/JP/JP.so: no space left on device

One workaround is to allocate a persistent_disk to the slave.

Is this the correct approach? Or should we include a docker job inside the jenkins-boshrelease that stores the docker graph on the ephemeral disks?