[Request]: system property for default `--dns`.

### Feature or enhancement request details

NOTE: Depends on #1425; there's no point tackling this until the TOML configuration is in place.

We've a number of issues relating to DNS resolution from containers not working properly when a third party DNS server conflicts with the server that macOS mDNSResponder wants to start.

Until an OS-level solution can be provided, it would be useful to have a system property (and potentially environment variables if session level overrides seem sensible) that allows the user to configure default values for `--dns`.

We can also consider properties for the other DNS CLI options here as well.

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Start by reviewing the TOML configuration work in #1425, then inspect how the `--dns` CLI option is defined and how system properties are expected to be represented. Done means an agreed configuration mechanism supplies a default for `--dns`, with the scope of environment-variable and other DNS-option overrides resolved.

Written by the indexing model from the issue text.