`/vendor` in `.gitignore` breaks Rails 7 deploys

[andyw8]

By default, a new Rails 7 app will create:

vendor/.keep
vendor/javascript/.keep

.gitignore has a /vendor entry (since #596) which prevents these from being committed.

The second path is referenced in app/assets/config/manifest.js. When that path doesn't exist, deploys will fail with:

Sprockets::ArgumentError: link_tree argument must be a directory
/[...]/releases/20220430191140/app/assets/config/manifest.js:4