WIP allow chroot instead of pivot_root

TODO: make this configurable or make it a fallback

pivot_root is not allowed in containers

src/libstore/build.cc

@@ -2616,11 +2616,6 @@ void DerivationGoal::runChild()
                 throw SysError("unable to make '/' private mount");
             }
 
-            /* Bind-mount chroot directory to itself, to treat it as a
-               different filesystem from /, as needed for pivot_root. */
-            if (mount(chrootRootDir.c_str(), chrootRootDir.c_str(), 0, MS_BIND, 0) == -1)
-                throw SysError(format("unable to bind mount '%1%'") % chrootRootDir);
-
             /* Set up a nearly empty /dev, unless the user asked to
                bind-mount the host /dev. */
             Strings ss;
@@ -2724,23 +2719,11 @@ void DerivationGoal::runChild()
             }
 
             /* Do the chroot(). */
-            if (chdir(chrootRootDir.c_str()) == -1)
-                throw SysError(format("cannot change directory to '%1%'") % chrootRootDir);
-
-            if (mkdir("real-root", 0) == -1)
-                throw SysError("cannot create real-root directory");
-
-            if (pivot_root(".", "real-root") == -1)
-                throw SysError(format("cannot pivot old root directory onto '%1%'") % (chrootRootDir + "/real-root"));
-
-            if (chroot(".") == -1)
+            if (chroot(chrootRootDir.c_str()) == -1)
                 throw SysError(format("cannot change root directory to '%1%'") % chrootRootDir);
 
-            if (umount2("real-root", MNT_DETACH) == -1)
-                throw SysError("cannot unmount real root filesystem");
-
-            if (rmdir("real-root") == -1)
-                throw SysError("cannot remove real-root directory");
+            if (chdir("/") == -1)
+                throw SysError(format("cannot change directory to '/'") % chrootRootDir);
 
             /* Switch to the sandbox uid/gid in the user namespace,
                which corresponds to the build user or calling user in