limit privileged flag for bgp container (#14932)

Why I did it
HLD implementation: Container Hardening (sonic-net/SONiC#1364)

Work item tracking
Microsoft ADO (number only): 14807420
How I did it
Reduce linux capabilities in privileged flag, retain NET_ADMIN and SYS_ADMIN capabilities

How to verify it
Install new image to DUT, verify bgp container is up
Run bgp sonic-mgmt kvmtest

rules/docker-fpm-frr.mk

@@ -28,7 +28,7 @@ SONIC_DOCKER_IMAGES += $(DOCKER_FPM_FRR)
 SONIC_DOCKER_DBG_IMAGES += $(DOCKER_FPM_FRR_DBG)
 
 $(DOCKER_FPM_FRR)_CONTAINER_NAME = bgp
-$(DOCKER_FPM_FRR)_RUN_OPT += --privileged -t
+$(DOCKER_FPM_FRR)_RUN_OPT += -t --cap-add=NET_ADMIN --cap-add=SYS_ADMIN
 $(DOCKER_FPM_FRR)_RUN_OPT += -v /etc/sonic:/etc/sonic:ro
 $(DOCKER_FPM_FRR)_RUN_OPT += -v /etc/timezone:/etc/timezone:ro