Use the same version of the dask image as the local version of distributed

[matyasselmeci]

There can be version incompatibilities between the dask-scheduler and the dask-worker, which lead to difficult-to-diagnose errors. Instead of using daskdev/dask:latest for the default Docker image for the workers, use daskdev/dask:$VERSION where $VERSION comes from:

from distributed.versions import get_versions; get_versions()

get_versions() might have a leading 0 in the month, whereas the image tag doesn't (and dask-scheduler --version doesn't).