This neatly gets rid of all the #:container? arguments. That's very
nice! :-) I haven't actually built and tested these patches, but these
LGTM. Just one minor observation below.

> +  (cons (service system-service-type
> +                 (let ((locale (operating-system-locale-directory os)))
> +                   (with-monad %store-monad
> +                     (return `(("locale" ,locale))))))
> +        (append base (list %containerized-shepherd-service))))

Why not rewrite this using just a call to append, that is remove the
call to cons? Like so:

(append base
        (list (service system-service-type
                       (let ((locale (operating-system-locale-directory os)))
                         (with-monad %store-monad
                                     (return `(("locale" ,locale))))))
              %containerized-shepherd-service))

Or perhaps, this can be done with cons* also.