Forge promotions and maven repo URLs are configurable (#3587)

docs/types-and-platforms/server-types/forge.md

@@ -45,6 +45,11 @@ In both of the cases above, there is no need for the `VERSION` or `FORGE_VERSION
 
     If an error occurred while installing Forge, it might be possible to resolve by temporarily setting `FORGE_FORCE_REINSTALL` to "true". Be sure to remove that variable after successfully starting the server.
 
+URLs configurable via environment variables:
+
+- `FORGE_PROMOTIONS_URL`: default is https://files.minecraftforge.net/net/minecraftforge/forge/promotions_slim.json
+- `FORGE_MAVEN_REPO_URL`: default is https://maven.minecraftforge.net
+
 ## Alternatives
 
 ### NeoForge