We introduce a new filter to implement custom commands in arbitrary containers.
It becomes easy to write convenient ad-hoc commands that users will
then be able to run either on Kubernetes or locally using a documented CLI.
Pluggable jobs are declared as Click commands and are responsible for
parsing their own arguments. See the new CLI_DO_COMMANDS filter.
Close https://github.com/overhangio/2u-tutor-adoption/issues/75
