Add ping package to pmon docker (#11550)

ping command is not working inside PMON docker (bullseye)
Use case: chassisd checks for module reachability inside PMON for "show chassis modules midplane-status" CLI, and on Cisco chassis, this uses ping command to check network reachability

dockers/docker-platform-monitor/Dockerfile.j2

@@ -25,7 +25,8 @@ RUN apt-get update &&   \
         i2c-tools       \
         psmisc          \
         python3-jsonschema \
-        libpci3
+        libpci3         \
+        iputils-ping
 
 # On Arista devices, the sonic_platform wheel is not installed in the container.
 # Instead, the installation directory is mounted from the host OS. However, this method