Run db_migrator for non first-time reboots (sonic-net#16116)

- Why I did it
The recent change sonic-net#15685 (comment) removed the db migration for non first reboots.
This is problematic for many deployments which doesn't rely on ZTP and push a custom config_db.json
Port to older branches after sonic-net#15685 is ported back

- How I did it
Re-introduce the logic to run the db_migrator on non-first boots

- How to verify it
Verified reboot and warm-reboot cases

Signed-off-by: Vivek Reddy Karri <vkarri@nvidia.com>

files/build_templates/docker_image_ctl.j2

@@ -257,9 +257,18 @@ function postStartAction()
             # This flag will be set to "1" after DB migration/initialization is completed as part of config-setup
             $SONIC_DB_CLI CONFIG_DB SET "CONFIG_DB_INITIALIZED" "0"
         else
+            $SONIC_DB_CLI CONFIG_DB SET "CONFIG_DB_INITIALIZED" "0"
+            # this is not a first time boot to a new image. Datbase container starts w/ old pre-existing config
+            if [[ -x /usr/local/bin/db_migrator.py ]]; then
+                # Migrate the DB to the latest schema version if needed
+                if [ -z "$DEV" ]; then
+                    /usr/local/bin/db_migrator.py -o migrate
+                fi
+            fi
             # set CONFIG_DB_INITIALIZED to indicate end of config load and migration
             $SONIC_DB_CLI CONFIG_DB SET "CONFIG_DB_INITIALIZED" "1"
         fi
+
         # Add redis UDS to the redis group and give read/write access to the group
         REDIS_SOCK="/var/run/redis${DEV}/redis.sock"
     else