machine: ssh: <machine-ssh-configuration> can include the host key.

* gnu/machine/ssh.scm (<machine-ssh-configuration>)[host-key]: New field.
(machine-ssh-session): Pass #:host-key to 'open-ssh-session'.
* doc/guix.texi (Invoking guix deploy): Document it.

doc/guix.texi

@@ -26447,6 +26447,18 @@ keyring.
 @item @code{identity} (default: @code{#f})
 If specified, the path to the SSH private key to use to authenticate with the
 remote host.
+
+@item @code{host-key} (default: @code{#f})
+This should be the SSH host key of the machine, which looks like this:
+
+@example
+ssh-ed25519 AAAAC3Nz@dots{} root@@example.org
+@end example
+
+When @code{host-key} is @code{#f}, the server is authenticated against
+the @file{~/.ssh/known_hosts} file, just like the OpenSSH @command{ssh}
+client does.
+
 @end table
 @end deftp
 

gnu/machine/ssh.scm

@@ -54,6 +54,7 @@
             machine-ssh-configuration-authorize?
             machine-ssh-configuration-port
             machine-ssh-configuration-user
+            machine-ssh-configuration-host-key
             machine-ssh-configuration-session))
 
 ;;; Commentary:
@@ -87,6 +88,8 @@
   (identity       machine-ssh-configuration-identity       ; path to a private key
                   (default #f))
   (session        machine-ssh-configuration-session        ; session
+                  (default #f))
+  (host-key       machine-ssh-configuration-host-key       ; #f | string
                   (default #f)))
 
 (define (machine-ssh-session machine)
@@ -98,11 +101,13 @@ one from the configuration's parameters if one was not provided."
         (let ((host-name (machine-ssh-configuration-host-name config))
               (user (machine-ssh-configuration-user config))
               (port (machine-ssh-configuration-port config))
-              (identity (machine-ssh-configuration-identity config)))
+              (identity (machine-ssh-configuration-identity config))
+              (host-key (machine-ssh-configuration-host-key config)))
           (open-ssh-session host-name
                             #:user user
                             #:port port
-                            #:identity identity)))))
+                            #:identity identity
+                            #:host-key host-key)))))
 
 
 ;;;