Ensure two containers cannot use the same DNS hostname. (#490)

- Closes #150, #394.
- Introduces `AttachmentConfiguration` type so that we can add key-value
options to `--network` in the future.
- Eliminates redundant `ContainersService.Item` type.
- Since we now ensure at ContainersService that hostnames will not
conflict, the network helper IP allocator now simply provides the
existing IP if for an allocation on an existing hostname, which should
handle (in an eventually consistent way) the case where a container
fails to deallocate an IP on shutdown.

Sources/Services/ContainerNetworkService/AllocationOnlyVmnetNetwork.swift

@@ -55,7 +55,6 @@ public actor AllocationOnlyVmnetNetwork: Network {
         guard case .created(let configuration) = _state else {
             throw ContainerizationError(.invalidState, message: "cannot start network \(_state.id) in \(_state.state) state")
         }
-        var defaultSubnet = "192.168.64.1/24"
 
         log.info(
             "starting allocation-only network",
@@ -65,21 +64,16 @@ public actor AllocationOnlyVmnetNetwork: Network {
             ]
         )
 
-        if let suite = UserDefaults.init(suiteName: UserDefaults.appSuiteName) {
-            // TODO: Make the suiteName a constant defined in DefaultsStore and use that.
-            // This will need some re-working of dependencies between NetworkService and Client
-            defaultSubnet = suite.string(forKey: "network.subnet") ?? defaultSubnet
-        }
-
-        let subnet = try CIDRAddress(defaultSubnet)
-        let gateway = IPv4Address(fromValue: subnet.lower.value + 1)
-        self._state = .running(configuration, NetworkStatus(address: subnet.description, gateway: gateway.description))
+        let subnet = DefaultsStore.get(key: .defaultSubnet)
+        let subnetCIDR = try CIDRAddress(subnet)
+        let gateway = IPv4Address(fromValue: subnetCIDR.lower.value + 1)
+        self._state = .running(configuration, NetworkStatus(address: subnetCIDR.description, gateway: gateway.description))
         log.info(
             "started allocation-only network",
             metadata: [
                 "id": "\(configuration.id)",
                 "mode": "\(configuration.mode)",
-                "cidr": "\(defaultSubnet)",
+                "cidr": "\(subnet)",
             ]
         )
     }