feat: Add ENTRYPOINT and tini to support Dask KubeCluster

[matthewfeickert]

To better support custom initalization of Dask KubeCluster container args, switch to using an ENTRYPOINT with tini.

For Dask KubeCluster, to customize the arguments passed to the container, spec["spec"]["scheduler"]["spec"]["containers"][0]["args"] (or "worker") expects that the args are able to be given directly as the container CMD without any additional setup. To ensure environmental setup in advance, an ENTRYPOINT should be used instead, as is done in https://github.com/dask/dask-docker/blob/fcb9fb839fb1085714b2ee4cbaca1c910ded5bba/base/Dockerfile#L32.

* Use an ENTRYPOINT to ensure /release_setup.sh is sourced to provide the
  correct environment.
   - For customized container commands to be given to Dask KubeClusters the
     args need to be able to be passed as CMD without additional environmental
     setup or an additional shell being spawned.
   - c.f. https://kubernetes.dask.org/en/latest/operator_kubecluster.html#customising-your-cluster
* Use tini to better support and guard this behavior.
   - c.f. https://github.com/krallin/tini