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Containers are actually resource constraints, and were first introduced in Solaris 10, along with zones, which is what you appear to understand under containers. Even on GNU/Linux, containers are implemented via cgroups, which are resource controls and not a virtualization solution.

On SmartOS, when you provision a zone, you get a fully virtualized UNIX server, and you can apply a container to it by defining resource constraints, but that is both pointless and unnecessary there. Once you have a fully virtualized server provisioned from an image with the imgadm(1M) and vmadm(1M), it is only logical that you will want to service individual components via pkg_rm and pkg_add, rather than baking an entire image all over again, and redeploying it, all over again. It's the rule of modularity: "write simple parts connected by clean interfaces" [Kernighan-Plaguer], and it applies particularly well to lightweight virtualization.