Mark distDir with CACHEDIR.TAG so we can easily exclude it from backups

[reitzig]

Goals

1. Help keep backup sizes down
2. Create a benefit that does not require user interaction

Non-Goals

1. Change any functionality of next.js

Background

distDir, specifically its subfolder cache, can become rather large, but does not seem to be worth backing up, usually, as its content can (and is, often) easily recreated.

bford.info/cachedir proposes a simple convention: add a file named CACHEDIR.TAG to directories that are, well caches.

This approach has been widely adopted, well beyond the examples given on that website. Modern examples include Rust/Cargo (which add the tag to several directories) and Borg Backup, which has a dedicated flag to ignore thus-marked directories.

Adding CACHEDIR.TAG to distDir manually does not work, as next build removes it.

Adding cache/CACHEDIR.TAG to distDir manually seems to work. (but see #6009)

Proposal

Ensure that distDir (default: .next) always contains CACHEDIR.TAG.

Failing that, ensure that <distDir>/cache always contains CACHEDIR.TAG.