Allow Fabric meta API to be configurable

For a few days now, I have been experiencing problems reaching https://meta.fabricmc.net/. This endpoint is accessed when starting my Minecraft server (TYPE=FABRIC). At first, I assumed a misconfiguration in my Docker network, but even when I try to reach the endpoint directly using curl, I intermittently encounter issues. The TLS handshake usually succeeds, but after that the connection either becomes extremely slow or the request times out entirely.

At this point, I can also rule out my home network and ISP as the cause. I am able to reproduce the issue on my laptop as well (connected via mobile data, no VPN).

I have tested several online curl tools available on the internet, and the request works without any issues there. This leads me to suspect that this might be a regional issue (Germany?). All resolved IP addresses belong to Cloudflare.

In the Discord of “The Fabric Project”, I coincidentally came across a similar domain:
https://meta2.fabricmc.net/

From a content perspective, the data appears to be identical. However, this domain resolves to an IP address located in Germany, which makes it seem like some kind of mirror or clone.

This leads to my question: would it be possible to override the URL via an environment variable, so that in my case I could fall back to using https://meta2.fabricmc.net/
instead?

Additionally, I find it strange that I can barely find any information about this issue online. This surely cannot be affecting only me?

Thanks in advance, and best regards.

_Originally posted by @janhuddel in https://github.com/itzg/docker-minecraft-server/issues/3331#issuecomment-3786974021_

Start by reviewing the merged pull request #3917 and the startup path that accesses meta.fabricmc.net for TYPE=FABRIC. Confirm whether the endpoint can be overridden through an environment variable, and verify that the configured URL is used successfully when starting a Fabric server.

Written by the indexing model from the issue text.