## TL;DR

Fixed by contributor dimkl in @clerk/nextjs 4.21.11 and enabled by default in 4.23.0, so no environment variable is needed anymore (the earlier workaround was CLERK_TRUST_HOST=true). Lesson: upgrade @clerk/nextjs to >= 4.23.0 when running behind Docker/reverse proxies.

## Steps

1. When Next.js runs inside a Docker container, authMiddleware builds redirect_url using YOUR_HOST:3000 instead of the container's actual host, breaking sign-in redirects.

## When to use

Use this skill when you run into "`authMiddleware` builds `redirect_url` using `YOUR_HOST:3000` when NextJS runs inside Docker container".

## When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

## Versions

Versions mentioned in the source: clerknextjs 4.21.11, 4.23.0. If you are on something much newer or older, the details may have shifted.

## Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.