[docker]: Add DHCP relay docker to generic build (#207)

Author: jleveque

platform/generic/rules.mk

@@ -3,4 +3,5 @@ SONIC_ALL += $(DOCKER_DATABASE) \
          $(DOCKER_TEAMD) \
          $(DOCKER_LLDP_SV2) \
          $(DOCKER_SNMP_SV2) \
-         $(DOCKER_PLATFORM_MONITOR)
+         $(DOCKER_PLATFORM_MONITOR) \
+         $(DOCKER_DHCP_RELAY)

rules/docker-dhcp-relay.mk

@@ -0,0 +1,7 @@
+# docker dhcp relay image
+
+DOCKER_DHCP_RELAY = docker-dhcp-relay.gz
+$(DOCKER_DHCP_RELAY)_PATH = $(DOCKERS_PATH)/docker-dhcp-relay
+$(DOCKER_DHCP_RELAY)_LOAD_DOCKERS = $(DOCKER_BASE)
+
+SONIC_SIMPLE_DOCKER_IMAGES += $(DOCKER_DHCP_RELAY)