Add 'One Image' implementation for DHCP relay (hopefully fix builds)

Author: Joe LeVeque

files/build_templates/dhcp_relay.service.j2

@@ -0,0 +1,12 @@
+[Unit]
+Description=DHCP relay container
+Requires=docker.service
+After=docker.service
+
+[Service]
+User={{ sonicadmin_user }}
+ExecStart=/usr/bin/{{ docker_container_name }}.sh start
+ExecStop=/usr/bin/{{ docker_container_name }}.sh stop
+
+[Install]
+WantedBy=multi-user.target