Resolve the given URI to a local file in the given docroot.

This is Filename.concat docroot (normalise uri). It decodes percent-encoding once and removes .. segments so that the request cannot escape the docroot. The returned filepath is fully qualified iff the given docroot is fully qualified.

normalise uri returns the path of uri with percent-encoding decoded exactly once and ./.. segments resolved, as a relative path that can never ascend above its root.

Use this to derive a lookup key from a request URI, for example a filesystem path or a key-value store key.