[docker-syncd]: Add optional shm-size to syncd container (#10516)

Why I did it
In the bringup of tomahawk4/trident4, we realized that such chips need a larger size of /dev/shm in syncd container, so we added the option --shm-size to the docker create for syncd. The default value for shm-size is 64m; after this change, people can add SYNCD_SHM_SIZE=128m to platform_env.conf to change it to 128m.

How to verify it
We verified that after this change, 1) on existing platforms without platform_env.conf, the size of /dev/shm in syncd container (df -h | grep shm) is still the default 64M; 2) after we add SYNCD_SHM_SIZE=128m to platform_env.conf, /dev/shm in syncd becomes 128M.

files/build_templates/docker_image_ctl.j2

@@ -233,6 +233,11 @@ start() {
         source $ASIC_CONF
     fi
 
+    PLATFORM_ENV_CONF=/usr/share/sonic/device/$PLATFORM/platform_env.conf
+    if [ -f "$PLATFORM_ENV_CONF" ]; then
+        source $PLATFORM_ENV_CONF
+    fi
+
     {%- if docker_container_name == "database" %}
     # Don't mount HWSKU in {{docker_container_name}} container.
     HWSKU=""
@@ -421,6 +426,7 @@ start() {
 {%- endif %}
 {%- if sonic_asic_platform == "broadcom" %}
 {%- if docker_container_name == "syncd" %}
+        --shm-size=${SYNCD_SHM_SIZE:-64m} \
         -v /var/run/docker-syncd$DEV:/var/run/sswsyncd \
 {%- endif %}
 {%- endif %}