Add script to start Docker test instances

This is an implementation of our Omnibus Docker install instructions, with slight modifications to adapt them to the support reality of:

• needing to sometimes change a small part of the template
• needing to avoid port-conflicts with other instances (Or is this just happening for me? 😅 )

@cmutua & @myacksmith: Could I please ask you to review, since you gave me useful tips about this in the last few days?

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I couldn't properly test this end-to-end yet, because of Docker/Podman troubles. Will try @dnsmichi's instructions here... Or these?