config.json is generated in the world-readable Nix store, but the
description did not warn against registry credentials or proxy
passwords. Point at Docker credential stores instead, note that docker
login still reports an error when it adds a registry to the read-only
file, and mention the helper's own store command. Also close the
unterminated link to the config.json reference.
This change converts redundant attribute assignments of the form `a =
a;` or `a = someSet.a;` into cleaner `inherit` statements. This reduces
verbosity and follows common Nix style for bringing attributes into
scope.
Statix Codes: W03 (manual_inherit), W04 (manual_inherit_from)
Also include statix and the rule in our configuration.
