Require only build directory to be writeable for build (#20977)
Require only build directory to be writeable for build (#20977)

Allows building without permission to write to the application directory, useful for containers and sandboxes.
JeromeFitz/next.js,flybayer/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,zeit/next.js,azukaru/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,zeit/next.js,zeit/next.js