[tracker-oci-images] Create 'tracker' user in the Ubuntu base image

[Sam Thursfield <sthursfield src gnome org>]

commit 15a622e40384a912af56a0dae8960c648ef16ff3
Author: Sam Thursfield <sam afuera me uk>
Date:   Fri Dec 13 23:36:14 2019 +0100

    Create 'tracker' user in the Ubuntu base image

 debian/prepare.sh | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/debian/prepare.sh b/debian/prepare.sh
index a926492..518cdb8 100644
--- a/debian/prepare.sh
+++ b/debian/prepare.sh
@@ -9,3 +9,6 @@ apt-get -yq build-dep tracker tracker-miners
 
 # For rss miner, which I guess Debian/Ubuntu disable by default
 apt-get -yq install libgrss-dev
+
+# We need to be able to run tests as a normal user, not 'root'.
+useradd -Um tracker