WP-CLI cannot write to its own cache directory

[johnbillion]

With a vanilla Altis local-server configuration, WP-CLI doesn't have permission to write to its own cache directory. This results in warnings when running a WP-CLI command that would normally cause it to write to its cache.

Steps to reproduce

1. Set up an Altis local-server site
2. Run composer server cli -- theme install twentytwenty
3. Observe that the command succeeds but produces a warning about the cache directory not being writable:

Warning: Failed to create directory '/var/www/.wp-cli/cache/': mkdir(): Permission denied.

[johnbillion]

Same issue reported here: WordPress/wpdev-docker-images#122