scripts/environment: Do not attempt to map GID 0.

* guix/scripts/environment.scm (launch-environment/container): Do not attempt
to map GID 0.

Fixes: #4234
Change-Id: If196430de5f09932e5dd736a55b75bbcda8cd7be

gnu/tests/base.scm

@@ -1168,6 +1168,21 @@ to supplementary group ~a...~%" other)
                                hello))
                          #$marionette))
 
+
+      ;; Check that guix-daemon is up and running and that the build
+      ;; environment is properly set up (build users, etc.).
+      (test-equal "containerized guix shell"
+        0
+        (marionette-eval
+         '(system "guix shell --no-cwd --bootstrap -C hello -- hello")
+         #$marionette))
+
+      (test-equal "containerized guix shell, unprivileged user"
+        0
+        (marionette-eval
+         '(system "su - user -c 'guix shell --bootstrap -C hello -- hello'")
+         #$marionette))
+
       (test-equal "guix build hello"
         0
         ;; Check that guix-daemon is up and running and that the build

guix/scripts/environment.scm

@@ -828,7 +828,11 @@ WHILE-LIST."
      (let* ((cwd      (getcwd))
             (home     (getenv "HOME"))
             (uid      (if user 1000 (getuid)))
-            (gid      (if user 1000 (getgid)))
+            (gid      (if user
+                          1000
+                          (match (getgid)
+                            (0 1000)
+                            (gid gid))))
 
             ;; On a foreign distro, the name service switch might be
             ;; dysfunctional and 'getpwuid' throws.  Don't let that hamper