Add entity prototype save test (#10274)

Content.Shared/Disposal/Components/SharedDisposalUnitComponent.cs

@@ -6,6 +6,8 @@ namespace Content.Shared.Disposal.Components
     [NetworkedComponent]
     public abstract class SharedDisposalUnitComponent : Component
     {
+        public const string ContainerId = "disposal-unit";
+
         // TODO: Could maybe turn the contact off instead far more cheaply as farseer (though not box2d) had support for it?
         // Need to suss it out.
         /// <summary>