Fix: Add explicit host entries to container configuration

[mazdak]

Type of Change

• Bug fix

Motivation and Context

While building a Docker Compose-like plugin for container and validating it against our real Docker Compose workload, we hit a core limitation: there was no way for callers to ask the runtime to append explicit entries to a container's /etc/hosts.

That showed up most clearly with Compose extra_hosts, especially the common host.docker.internal pattern. The plugin could parse those mappings, but there was no core field to carry them into the sandbox, so the runtime always generated only the default localhost/container-name entries.

In practice, containers that depended on host aliases still failed name resolution even though the compose file specified them.

Why this belongs in core

This is not something the plugin can fake safely. /etc/hosts is generated in the sandbox layer, so callers need a first-class way to provide additional host entries to the runtime.

What this changes

• add ContainerConfiguration.HostEntry
• add ContainerConfiguration.hosts
• preserve those entries through configuration encoding/decoding
• extend sandbox host generation to append caller-provided host entries after the default localhost and primary hostname entries
• add focused tests for round-tripping and host resolution behavior

Testing

• Tested locally
• Added/updated tests

[JaewonHur]

Why are we splitting by / here?

public struct IPv4Address {
    @inlinable
    public var description: String {
        "\(bytes[0]).\(bytes[1]).\(bytes[2]).\(bytes[3])"
    }
}

[vxnkjyffmq-code]

We are splitting by / here because the value coming in as primaryAddress is not a plain IP address — it is a CIDR notation string (e.g. "192.168.1.45/24").
The /24 part is the subnet mask. We only want the actual IP address (192.168.1.45), so we split on the / and take the first part.
Cleaned-up & safer version of that function:

extension SandboxService {
static func resolvedHosts(
hostname: String,
primaryAddress: String?,
extraHosts: [ContainerConfiguration.HostEntry]
) -> [ContainerConfiguration.HostEntry] {

    var hosts: [ContainerConfiguration.HostEntry] = [
        ContainerConfiguration.HostEntry(ipAddress: "127.0.0.1", hostnames: ["localhost"])
    ]

    if let primaryAddress {
        // Split off the CIDR suffix if present (e.g. "192.168.1.45/24" → "192.168.1.45")
        let ipOnly = primaryAddress.split(separator: "/").first.map(String.init) ?? primaryAddress
        
        hosts.append(
            ContainerConfiguration.HostEntry(
                ipAddress: ipOnly,
                hostnames: [hostname]
            )
        )
    }

    // Add any extra hosts passed in
    hosts.append(contentsOf: extraHosts)

    return hosts
}

}

[vxnkjyffmq-code]

Why are we splitting by / here?

public struct IPv4Address {
    @inlinable
    public var description: String {
        "\(bytes[0]).\(bytes[1]).\(bytes[2]).\(bytes[3])"
    }
}

Also with the way I am running low latency it must be a steady string no breaking if possible.! I can show you diagrams or math that proves it.

[JaewonHur]

It'd be good to have a follow up PR that wires this to CLI (e.g., --add-host [name:ip] in Docker).