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

[katiewasnothere]

Today there is a subtle difference between what the default DNS settings are for container aka the apiserver itself, which would determine the hostname resolution (aka the A record) for a given container on the host, and the DNS settings for setting up the resolv.conf in a container. I think we should maintain this distinction. There could be scenarios where a user does not want the default DNS domain on the host to necessarily match the default DNS domain that the container application uses in the container itself.

I think we should add a new field on the ContainerConfig type that has the default DNS settings for running a container.

Essentially we'd end up with something like this in the TOML:

[dns] <-------- default DNS settings for the APIServer
domain = "test"

[container.dns] <----------- default DNS settings for containers 
server = "8.8.8.8"
domain = "foo"
search = ["foo", "test"]
options = ["haha"]

[0xMH]

Thanks, good insight. I removed that extra builder startup call.

[github-actions]

Code Coverage

Tier	Line Coverage
Unit	34.55%
Integration	19.69%
Combined	53.62%