backup: Dereference symlinks when building tarball.

In docker-zulip installs, /etc/zulip/zulip.conf,
/etc/zulip/zulip-secrets.conf, and /home/zulip/uploads are all
symlinks into the `/data` directory which is mounted as a Docker
Volume.  By default, `tar` does not dereference symlinks, leading to
backups that are missing these critical pieces.

Add `-h` to the `tar` invocation, to follow symlinks, so backups in
Docker have all of their pieces.  Since none of the contents of the
backup intentionally use symlinks, this is safe.

Co-authored-by: Alex Vandiver <alexmv@zulip.com>
(cherry picked from commit d4b30e9e9d)

zerver/management/commands/backup.py

@@ -133,7 +133,7 @@ class Command(ZulipBaseCommand):
                     [
                         "tar",
                         f"--directory={tmp}",
-                        "-cPzf",
+                        "-cPhzf",
                         tarball_path,
                         *transform_args,
                         "--",