[202012] dhcprelay add Loopback0 as source interface in dualtor scenario (sonic-net#15744)

#### Why I did it
1. sonic-build image side change to fix source interface selection in dual tor scenario.
dhcprelay related PR:
sonic-net/sonic-dhcp-relay#40

2. Announce dhcprelay submodule to 3578eb3 (to invoke [#40](sonic-net/sonic-dhcp-relay#40
) PR)

##### Work item tracking
- Microsoft ADO 24243215

#### How to verify it
run test case, dhcp_relay/test_dhcpv6_relay.py

Author: jcaiMR

dockers/docker-dhcp-relay/dhcpv6-relay.agents.j2

@@ -13,7 +13,7 @@
 [program:dhcp6relay]
 command=/usr/sbin/dhcp6relay
 {#- Dual ToR Option #}
-{% if 'subtype' in DEVICE_METADATA['localhost'] and DEVICE_METADATA['localhost']['subtype'] == 'DualToR' %} -d{% endif %}
+{% if 'subtype' in DEVICE_METADATA['localhost'] and DEVICE_METADATA['localhost']['subtype'] == 'DualToR' %} -u Loopback0 {% endif %}
 
 priority=3
 autostart=false