Rebuild the image as a rock

[dwilding]

I think we should rebuild the demo server as a rock image.

Follow-up actions:

• Check that the image works with charms deployed in a Multipass VM on ARM Macs
• Update the version number of the image in From zero to hero: Write your first Kubernetes charm chapters (Ops)
• Update the repo & version number in How to manage charms or bundles (Juju)