[feature] Add warning about trusted-proxies to make config easier

[tsmethurst]

Description

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This pull request adds a warning about trusted-proxies at the top of every GoToSocial web page if it looks as though the trusted-proxies value is incorrectly configured:

The warning directs users to a new documentation page that provides a tl;dr of setting trusted proxies correctly.

To make this even more foolproof, the docker-compose.yaml file has also been updated to use a network with a specific address, and to trust the gateway of that address by default.

Closes #3443

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.