There are essentially three ways to build your NodeMCU firmware: cloud build service, Docker image, dedicated Linux environment (possibly VM).

Cloud Build Service

NodeMCU "application developers" just need a ready-made firmware. There's a cloud build service with a nice UI and configuration options for them.

Docker Image

Occasional NodeMCU firmware hackers don't need full control over the complete tool chain. They might not want to setup a Linux VM with the build environment. Docker to the rescue. Give Docker NodeMCU build a try.

Linux Build Environment

NodeMCU firmware developers commit or contribute to the project on GitHub and might want to build their own full fledged build environment with the complete tool chain. There is a post in the esp8266.com Wiki that describes this.