Bug 1607713 - set GDM3 custom.conf to force use of X11 on linux1804 r=jmaher

Changes:

Add a new configuration file for GDM that forces the use of X11 instead of Wayland.

This may help with reducing the number of crashes due to window managers.

Differential Revision: https://phabricator.services.mozilla.com/D67006

--HG--
extra : moz-landing-system : lando

taskcluster/docker/ubuntu1804-test/Dockerfile

@@ -100,6 +100,8 @@ RUN     chmod +x /bin/taskcluster-interactive-shell
 # "Operation not permitted". Luckily it doesn't actually require the capability.
 RUN     setcap -r /usr/bin/gnome-keyring-daemon
 
+COPY custom.conf /etc/gdm3/custom.conf
+
 # Ubuntu 18.04 (or any other GNOME3 based systems) needs dbus to have various
 # test function as expected. Use entrypoint to initialize dbus as root.
 COPY dbus.sh /usr/local/bin/dbus.sh

taskcluster/docker/ubuntu1804-test/custom.conf

@@ -0,0 +1,28 @@
+# GDM configuration storage
+#
+# See /usr/share/gdm/gdm.schemas for a list of available options.
+
+[daemon]
+# Uncoment the line below to force the login screen to use Xorg
+WaylandEnable=false
+
+# Enabling automatic login
+#  AutomaticLoginEnable = true
+#  AutomaticLogin = user1
+
+# Enabling timed login
+#  TimedLoginEnable = true
+#  TimedLogin = user1
+#  TimedLoginDelay = 10
+
+[security]
+
+[xdmcp]
+
+[chooser]
+
+[debug]
+# Uncomment the line below to turn on debugging
+# More verbose logs
+# Additionally lets the X server dump core if it crashes
+#Enable=true