feat: Set network dynamically + docker improvements

[simonj2]

What

• Improves documentation
• Allows configuration of different URLs for querying the OFF API
• Switches to the default network, which was validated in testing

See corresponding server PR: https://github.com/openfoodfacts/openfoodfacts-server/pull/7239

[simonj2]

@alexgarel - Hey, do you feel okay to review this? I've validated it on my computer, but suspect we'll want to:

• Change the network name to webnet so it'll work in prod
• Deploy on an actual host
• Run the index process on the host
• Expose the URL to allow some manual testing

If that seems good, I can change the network name, but then I might need to work with you to do the subsequent steps?