Document pull of robotoff docker image to run locally

[alexgarel]

Is your feature request related to a problem? Please describe.

You can build the docker images for robotoff localy, and we usually do this in dev because we want some specific settings.
But sometimes you want a quick glimpse to reproduce a bug, etc... and it can be long to rebuild.

Describe the solution you'd like
In those case you should be able to pull the distant image from ghcr.io and run it locally.
For the dev it maybe better not to mess up with his own env, and use an alternative COMPOSE_PROJECT_NAME.

Additional context
This was mentioned here: #465 (comment)