[docker]: get hostname from config db instead of minigraph (#3004)

minigraph may not be always available on the some system configuration.
Should use config db as the source of truth.

Author: lguohan

files/build_templates/docker_image_ctl.j2

@@ -130,8 +130,8 @@ start() {
     {%- else %}
     # Obtain our HWSKU as we will mount directories with these names in each docker
     HWSKU=`sonic-cfggen -d -v 'DEVICE_METADATA["localhost"]["hwsku"]'`
+    HOSTNAME=`sonic-cfggen -d -v 'DEVICE_METADATA["localhost"]["hostname"]'`
     {%- endif %}
-    HOSTNAME=`sonic-cfggen -m -v 'DEVICE_METADATA["localhost"]["hostname"]'`
     if [ -z "$HOSTNAME" ] || ! [[ $HOSTNAME =~ ^[a-zA-Z0-9.\-]*$ ]]; then
         HOSTNAME=`hostname`
     fi