* gnu/system/vm.scm (expression->derivation-in-linux-vm)
[user-builder]: Define in non-monadic style as 'program-file'.
[loader]: Likewise, and 'execl' USER-BUILDER instead of loading it.
(system-docker-image): Pass BUILD as the second
argument to 'expression->derivation-in-linux-vm'.
(make-iso9660-image, qemu-image): Remove call to 'reboot'.