Switch Docker image to official one

Switching from from avhost/docker-matrix (silviof/docker-matrix)
to matrixdotorg/synapse.

The avhost/docker-matrix (silviof/docker-matrix) image used to bundle
in the coturn STUN/TURN server, so as part of the move,
we're separating this to a separately-ran service
(matrix-coturn.service, powered by instrumentisto/coturn-docker-image)

roles/matrix-server/templates/systemd/matrix-coturn.service.j2

@@ -0,0 +1,25 @@
+[Unit]
+Description=Matrix Coturn server
+After=docker.service
+Requires=docker.service
+
+[Service]
+Type=simple
+ExecStartPre=-/usr/bin/docker kill matrix-coturn
+ExecStartPre=-/usr/bin/docker rm matrix-coturn
+ExecStart=/usr/bin/docker run --rm --name matrix-coturn \
+			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			-p 3478:3478 \
+			-p 3478:3478/udp \
+			-p {{ matrix_coturn_turn_udp_min_port }}-{{ matrix_coturn_turn_udp_max_port }}:{{ matrix_coturn_turn_udp_min_port }}-{{ matrix_coturn_turn_udp_max_port }}/udp \
+			-v {{ matrix_synapse_config_dir_path }}:/matrix-config:ro \
+			-v {{ matrix_coturn_config_path }}:/turnserver.conf:ro \
+			{{ docker_coturn_image }} \
+			-c /turnserver.conf
+ExecStop=-/usr/bin/docker kill matrix-coturn
+ExecStop=-/usr/bin/docker rm matrix-coturn
+Restart=always
+RestartSec=30
+
+[Install]
+WantedBy=multi-user.target