Of course you can do that with juju. You can do that today, and will be able to continue to do that tomorrow. Nothing forces you to change anything. If you want to deploy a container and never touch it, just do it.
This is also very dogmatic, though, and as soon as you assign any kind of read-write storage to a pod you’re already breaking that rule, because the only reason to have read-write storage associated with a pod is if you want to make modifications to that environment at runtime. In fact, even if you don’t assign storage, but the software being deployed accepts dynamic changes via its API, you are also modifying that software.
This is the software deployment version of functional programming. Purity is beautiful, until you actually want side effects to do what in fact is essential. But juju is there for you in either case. If you don’t want to mutate your pod, just don’t.