Instead of doing the current hack of copying the agola toolbox inside the host
tmp dir (always done but only needed when running the executor inside a docker
container) that has different issues (like tmp file removal done by
tmpwatch/systemd-tmpfiles), use a solution similar to the k8s driver: for every
pod create a volume containing the agola-toolbox and remove it at pod removal.
We could also use a single "global" volume but we should handle cases like
volume removal (i.e. a docker volume prune command). So for now just create a
dedicated per pod volume.