Update the Docker installation

[jtpio]

We currently install docker using the tljh_extra_apt_packages hook:

tljh-voila-gallery/tljh-voila-gallery/tljh_voila_gallery/__init__.py

Lines 75 to 79 in f7bac2d

	@hookimpl
	def tljh_extra_apt_packages():
	return [
	'docker.io'
	]

However the Docker documentation recommends uninstalling docker.io and use a different procedure instead:

https://docs.docker.com/engine/install/ubuntu/

We could then add the steps to install docker-ce in the tljh_post_install hook instead:

tljh-voila-gallery/tljh-voila-gallery/tljh_voila_gallery/__init__.py

Lines 82 to 84 in f7bac2d

	@hookimpl
	def tljh_post_install():
	ensure_builder_units()

By executing apt commands directly, similar to what is done in TLJH:

https://github.com/jupyterhub/the-littlest-jupyterhub/blob/bdaab082ea1171b65a30bd93e27db4fc9b694a44/bootstrap/bootstrap.py#L127-L129