Fix Alpine DNS issues by falling back to host nameservers

[Ronitsabhaya75]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Resolves #1033.

Alpine Linux containers (using musl libc) often encounter transient DNS errors ("DNS: transient error (try again later)") during builds, particularly when running apk update.

This change modifies SandboxService to explicitly read nameservers from the host's /etc/resolv.conf and append them to the container's DNS configuration as a fallback.

    private func getDefaultNameservers(attachmentConfigurations: [AttachmentConfiguration]) async throws -> [String] {
        var nameservers: [String] = []

        for attachmentConfiguration in attachmentConfigurations {

            nameservers.append(status.ipv4Gateway.description)
            break
        }

        let systemNameservers = self.getSystemNameservers()
        for ns in systemNameservers {
            if !nameservers.contains(ns) {
                nameservers.append(ns)
            }
        }
        return nameservers
    }

Testing

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