linux-container: Mount a new /dev/pts instance in the container.

Fixes <https://bugs.gnu.org/36463>.
Reported by Steffen Rytter Postas <nc@scalehost.eu>.

* gnu/build/linux-container.scm (mount-file-systems): When /dev/ptmx
exists on the host, explicitly mount a new instance of devpts and make
/dev/ptmx a symlink to /dev/pts/ptmx.

gnu/build/linux-container.scm

@@ -130,9 +130,14 @@ for the process."
               "/dev/random"
               "/dev/urandom"
               "/dev/tty"
-              "/dev/ptmx"
               "/dev/fuse"))
 
+  ;; Mount a new devpts instance on /dev/pts.
+  (when (file-exists? "/dev/ptmx")
+    (mount* "none" (scope "/dev/pts") "devpts" 0
+            "newinstance,mode=0620")
+    (symlink "/dev/pts/ptmx" (scope "/dev/ptmx")))
+
   ;; Setup the container's /dev/console by bind mounting the pseudo-terminal
   ;; associated with standard input when there is one.
   (let* ((in      (current-input-port))