nixos/doc/imperative-containers: add `boot.enableContainers` note

Emily 42a1d455 6077545d

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nixos
doc
manual
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nixos/doc/manual/administration/imperative-containers.section.md
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We'll cover imperative container management using `nixos-container`
first. Be aware that container management is currently only possible as
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`root`.
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`root`, and that you need to enable [](#opt-boot.enableContainers) explicitly.
You create a container with identifier `foo` as follows: