Add TOML configuration for default DNS nameservers, search, options

[0xMH]

Type of Change

• New feature

Motivation and Context

Closes #1449. Adds default values for --dns, --dns-search, --dns-option, and --dns-domain to the [dns] section of ~/.config/container/config.toml, so users hitting macOS mDNSResponder conflicts can set the workaround once instead of repeating it on every invocation. Depends on the merged TOML configuration introduced by #1425.

Defaults are read by container run, container build, and container builder start via a shared Utility.dnsConfiguration(from:defaults:) helper. CLI flags take precedence; --no-dns still disables DNS. Two pre-existing bugs in the build path were fixed to make the feature work end-to-end: BuildCommand was forwarding only dnsNameservers to the builder (now forwards all four DNS fields), and BuilderStart's dnsChanged check only compared the first non-empty field (now compares all four).

Testing

• Tested locally
• Added/updated tests
• Added/updated docs