Activate direct asynchronous processes for Tramp container methods

* doc/misc/tramp.texi (Remote processes): Mention also
tramp-container.el.

* lisp/emacs-lisp/ert-x.el (ert-remote-temporary-file-directory):
Add `tramp-direct-async' to "mock" method.

* lisp/net/tramp-container.el
(tramp-methods) <docker, podman, kubernetes>: Add `tramp-direct-async'.

* lisp/net/tramp.el (tramp-methods): Adapt docstring.
(tramp-handle-make-process): Handle `tramp-direct-async'.

* test/lisp/net/tramp-tests.el (tramp-methods) <mock>:
Add `tramp-direct-async'.

doc/misc/tramp.texi

@@ -4253,8 +4253,9 @@ Furthermore, this approach has the following limitations:
 
 @itemize
 @item
-It works only for connection methods defined in @file{tramp-adb.el},
-@file{tramp-sh.el} and @file{tramp-sshfs.el}.
+It works only for some connection methods defined in
+@file{tramp-adb.el}, @file{tramp-container.el}, @file{tramp-sh.el} and
+@file{tramp-sshfs.el}.
 
 @item
 It does not support interactive user authentication.  With

lisp/emacs-lisp/ert-x.el

@@ -560,6 +560,7 @@ The same keyword arguments are supported as in
          '("mock"
 	   (tramp-login-program	     "sh")
 	   (tramp-login-args	     (("-i")))
+           (tramp-direct-async       ("-c"))
 	   (tramp-remote-shell	     "/bin/sh")
 	   (tramp-remote-shell-args  ("-c"))
 	   (tramp-connection-timeout 10)))

lisp/net/tramp-container.el

@@ -163,6 +163,7 @@ see its function help for a description of the format."
                                    ("-u" "%u")
                                    ("%h")
 			           ("%l")))
+		(tramp-direct-async (,tramp-default-remote-shell "-c"))
                 (tramp-remote-shell ,tramp-default-remote-shell)
                 (tramp-remote-shell-login ("-l"))
                 (tramp-remote-shell-args ("-i" "-c"))))
@@ -174,6 +175,7 @@ see its function help for a description of the format."
                                    ("-u" "%u")
                                    ("%h")
 			           ("%l")))
+		(tramp-direct-async (,tramp-default-remote-shell "-c"))
                 (tramp-remote-shell ,tramp-default-remote-shell)
                 (tramp-remote-shell-login ("-l"))
                 (tramp-remote-shell-args ("-i" "-c"))))
@@ -186,6 +188,7 @@ see its function help for a description of the format."
                                    ("--")
 			           ("%l")))
 		(tramp-config-check tramp-kubernetes--current-context-data)
+		(tramp-direct-async (,tramp-default-remote-shell "-c"))
                 (tramp-remote-shell ,tramp-default-remote-shell)
                 (tramp-remote-shell-login ("-l"))
                 (tramp-remote-shell-args ("-i" "-c"))))

lisp/net/tramp.el

@@ -278,8 +278,9 @@ pair of the form (KEY VALUE).  The following KEYs are defined:
 
   * `tramp-direct-async'
     Whether the method supports direct asynchronous processes.
-    Until now, just \"ssh\"-based, \"sshfs\"-based and
-    \"adb\"-based methods do.
+    Until now, just \"ssh\"-based, \"sshfs\"-based, \"adb\"-based
+    and container methods do.  If it is a list of strings, they
+    are used to construct the remote command.
 
   * `tramp-config-check'
     A function to be called with one argument, VEC.  It should
@@ -4804,7 +4805,14 @@ substitution.  SPEC-LIST is a list of char/value pairs used for
 	       (command
 	        (append
 		 `("cd" ,(tramp-shell-quote-argument localname) "&&" "(" "env")
-		 env `(,command ")"))))
+		 env `(,command ")")))
+		;; Add remote shell if needed.
+	       (command
+		(if (consp (tramp-get-method-parameter v 'tramp-direct-async))
+		    (append
+		     (tramp-get-method-parameter v 'tramp-direct-async)
+                     `(,(mapconcat #'identity command " ")))
+		  command)))
 
 	  ;; Check for `tramp-sh-file-name-handler', because something
 	  ;; is different between tramp-sh.el, and tramp-adb.el or

test/lisp/net/tramp-tests.el

@@ -140,10 +140,11 @@ A resource file is in the resource directory as per
        ((eq system-type 'windows-nt) null-device)
        (t (add-to-list
            'tramp-methods
-           '("mock"
-	     (tramp-login-program	"sh")
+           `("mock"
+	     (tramp-login-program	,tramp-default-remote-shell)
 	     (tramp-login-args		(("-i")))
-	     (tramp-remote-shell	"/bin/sh")
+             (tramp-direct-async	("-c"))
+	     (tramp-remote-shell	,tramp-default-remote-shell)
 	     (tramp-remote-shell-args	("-c"))
 	     (tramp-connection-timeout	10)))
           (add-to-list