Remove more defaults from CLI

[aronchick]

The current smallest possible command to execute on Bacalhau is the following:

 bacalhau docker run ubuntu -- /bin/true

Both docker and ubuntu should be defaults. Ideally, the smallest command should be:

bacalhau run /bin/true

• [ ] Default run to running docker unless --determinism flag is present
• [ ] Default image to be ubuntu (or other equivalent, whatever we feel like is appropriate)
• [ ] Maybe pre-cache a set of common images we recommend on every machine?

[lukemarsden]

bacalhau run /bin/true is a bit too magic IMO because /bin/true actually exists inside a default docker image, not the /bin/true on the system

I am open to some more autodetection though

[wdbaruni]

A default docker image, or any execution environment, might make sense not only to simplify user experience, but to also minimize users defining different images to execute simple tasks. This will reduce cost and latency in compute nodes and avoid having to pull new images for simple jobs.

If docker is the environment of choice, I would recommend creating our own image rather than ubuntu to be able to pack it with more features to enable different types of jobs, and to have more control on the image release cycle