environment: '-C' creates namespaces where the user is not root.

* guix/scripts/environment.scm (launch-environment/container): Add UID
and GID.  Use them in PASSWD and GROUPS.  Pass them as #:guest-uid and
 #:guest-gid to 'call-with-container'.
* tests/guix-environment-container.sh: Test the inner UID.
In '--user' test, replace hard-coded 0 with 1000.
* doc/guix.texi (Invoking guix environment): Adjust accordingly.

doc/guix.texi

@@ -4557,9 +4557,11 @@ Run @var{command} within an isolated container.  The current working
 directory outside the container is mapped inside the container.
 Additionally, unless overridden with @code{--user}, a dummy home
 directory is created that matches the current user's home directory, and
-@file{/etc/passwd} is configured accordingly.  The spawned process runs
-as the current user outside the container, but has root privileges in
-the context of the container.
+@file{/etc/passwd} is configured accordingly.
+
+The spawned process runs as the current user outside the container.  Inside
+the container, it has the same UID and GID as the current user, unless
+@option{--user} is passed (see below.)
 
 @item --network
 @itemx -N
@@ -4587,8 +4589,9 @@ the environment.
 @itemx -u @var{user}
 For containers, use the username @var{user} in place of the current
 user.  The generated @file{/etc/passwd} entry within the container will
-contain the name @var{user}; the home directory will be
-@file{/home/USER}; and no user GECOS data will be copied.  @var{user}
+contain the name @var{user}, the home directory will be
+@file{/home/@var{user}}, and no user GECOS data will be copied.  Furthermore,
+the UID and GID inside the container are 1000.  @var{user}
 need not exist on the system.
 
 Additionally, any shared or exposed path (see @code{--share} and

guix/scripts/environment.scm

@@ -459,17 +459,19 @@ will be used for the passwd entry.  LINK-PROFILE? creates a symbolic link from
     (return
      (let* ((cwd      (getcwd))
             (home     (getenv "HOME"))
+            (uid      (if user 1000 (getuid)))
+            (gid      (if user 1000 (getgid)))
             (passwd   (let ((pwd (getpwuid (getuid))))
                         (password-entry
                          (name (or user (passwd:name pwd)))
                          (real-name (if user
                                         ""
                                         (passwd:gecos pwd)))
-                         (uid 0) (gid 0) (shell bash)
+                         (uid uid) (gid gid) (shell bash)
                          (directory (if user
                                         (string-append "/home/" user)
                                         (passwd:dir pwd))))))
-            (groups   (list (group-entry (name "users") (gid 0))
+            (groups   (list (group-entry (name "users") (gid gid))
                             (group-entry (gid 65534) ;the overflow GID
                                          (name "overflow"))))
             (home-dir (password-entry-directory passwd))
@@ -541,6 +543,8 @@ will be used for the passwd entry.  LINK-PROFILE? creates a symbolic link from
              ;; A container's environment is already purified, so no need to
              ;; request it be purified again.
              (launch-environment command profile manifest #:pure? #f)))
+          #:guest-uid uid
+          #:guest-gid gid
           #:namespaces (if network?
                            (delq 'net %namespaces) ; share host network
                            %namespaces)))))))

tests/guix-environment-container.sh

@@ -44,6 +44,19 @@ else
     test $? = 42
 fi
 
+# By default, the UID inside the container should be the same as outside.
+uid="`id -u`"
+inner_uid="`guix environment -C --ad-hoc --bootstrap guile-bootstrap \
+  -- guile -c '(display (getuid))'`"
+test $inner_uid = $uid
+
+# When '--user' is passed, the UID should be 1000.  (Note: Use a separate HOME
+# so that we don't run into problems when the test directory is under /home.)
+export tmpdir
+inner_uid="`HOME=$tmpdir guix environment -C --ad-hoc --bootstrap guile-bootstrap \
+  --user=gnu-guix -- guile -c '(display (getuid))'`"
+test $inner_uid = 1000
+
 if test "x$USER" = "x"; then USER="`id -un`"; fi
 
 # Check whether /etc/passwd and /etc/group are valid.
@@ -123,7 +136,7 @@ rm $tmpdir/mounts
 
 # Test that user can be mocked.
 usertest='(exit (and (string=? (getenv "HOME") "/home/foognu")
-                     (string=? (passwd:name (getpwuid 0)) "foognu")
+                     (string=? (passwd:name (getpwuid 1000)) "foognu")
                      (file-exists? "/home/foognu/umock")))'
 touch "$tmpdir/umock"
 HOME="$tmpdir" guix environment --bootstrap --container --user=foognu \