system: Add 'essential-services' field to <operating-system>.

* gnu/system.scm (<operating-system>)[essential-services]: New field.
(operating-system-directory-base-entries): Remove #:container? keyword
and keep only the not-container branch.
(essential-services): Likewise.
(operating-system-services): Likewise, and call
'operating-system-essential-services' instead of 'essential-services'.
(operating-system-activation-script): Remove #:container?.
(operating-system-boot-script): Likewise.
(operating-system-derivation): Likewise.
* gnu/system/linux-container.scm (container-essential-services): New procedure.
(containerized-operating-system): Use it and set the
'essential-services' field.
(container-script): Remove call to 'operating-system-derivation'.
* gnu/system/vm.scm (system-docker-image): Likewise.
* doc/guix.texi (operating-system Reference): Document 'essential-services'.

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 37fef40522..7d80c00530 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -10531,6 +10531,13 @@ details.
 @item @code{services} (default: @var{%base-services})
 A list of service objects denoting system services.  @xref{Services}.
 
+@cindex essential services
+@item @code{essential-services} (default: ...)
+The list of ``essential services''---i.e., things like instances of
+@code{system-service-type} and @code{host-name-service-type} (@pxref{Service
+Reference}), which are derived from the operating system definition itself.
+As a user you should @emph{never} need to touch this field.
+
 @item @code{pam-services} (default: @code{(base-pam-services)})
 @cindex PAM
 @cindex pluggable authentication modules