`authMiddleware` builds `redirect_url` using `YOUR_HOST:3000` when NextJS runs inside Docker container
## Context
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.
## Verified resolution
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.