FR: Add njs support

[Tom Tang]

Official images have njs module built in since 2019. Is it possible to build it in this image?

njs is a subset of the JavaScript language that allows extending nginx functionality. njs is created in compliance with ECMAScript 5.1 (strict mode) with some ECMAScript 6 and later extensions. The compliance is still evolving.

[Maciej Brencz]

• Installation instructions
• Examples

[Maciej Brencz]

@qwerzl - added to the image. Enjoy :)

[Tom Tang]

Thanks a lot!