linux-container: Compute essential services for THIS-OPERATING-SYSTEM.

Previously, the 'essential-services' would correspond to the initial,
non-containerized OS.  Thus, all the file systems removed in
'container-essential-services' would actually still be there because the
essential services would be computed on the non-containerized OS.

This is a followup to 69cae3d3356a69b7fe69481338f760545995485e.

* gnu/system/linux-container.scm (container-essential-services): Call
'operating-system-default-essential-services' to get the baseline
services.
(containerized-operating-system): Pass THIS-OPERATING-SYSTEM, not OS, to
'container-essential-services'.
Add a dummy root file system to 'file-systems'.
(container-script)[mountable-file-system?]: New procedure.
Use it.

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