Perhaps, chrooted environment is required for effeciently serving static files, I don't know.

And of all apps - nginx, which has been designed to be self-contained (process-isolation is good enough) and has almost zero external dependencies (openssl and pcre, which are parts of any modern OS). I could understand the idea to "containerize" that pile of Java crap with its 100500 maven dependencies just in order not to touch it again, but nginx...

BTW, does Docker's "port mapping", which is, of course, has zero overhead, is what one needs in front of nginx in production?