[docker_image_ctl] flush ASIC_DB on fast boot (sonic-net#19106)

- Why I did it

I observed an issue with fast-reboot that in a rare circumstances a queued FDB event might be written to ASIC_DB by a thread inside syncd after a call to FLUSHDB ASIC_DB was made.
That left ASIC_DB only with one record about that FDB entry and caused syncd to crash at start:

Mar 15 13:28:42.765108 sonic NOTICE syncd#SAI: :- Syncd: syncd started
Mar 15 13:28:42.765268 sonic NOTICE syncd#SAI: :- onSyncdStart: performing hard reinit since COLD start was performed
Mar 15 13:28:42.765451 sonic NOTICE syncd#SAI: :- readAsicState: loaded 1 switches
Mar 15 13:28:42.765465 sonic NOTICE syncd#SAI: :- readAsicState: switch VID: oid:0x21000000000000
Mar 15 13:28:42.765465 sonic NOTICE syncd#SAI: :- readAsicState: read asic state took 0.000205 sec
Mar 15 13:28:42.766364 sonic NOTICE syncd#SAI: :- onSyncdStart: on syncd start took 0.001097 sec
Mar 15 13:28:42.766376 sonic ERR syncd#SAI: :- run: Runtime error during syncd init: map::at
Mar 15 13:28:42.766376 sonic NOTICE syncd#SAI: :- sendShutdownRequest: sending switch_shutdown_request notification to OA for switch: oid:0x0
Mar 15 13:28:42.766518 sonic NOTICE syncd#SAI: :- sendShutdownRequestAfterException: notification send successfully
The fix is done in utilities in fast-reboot script, however in order to allow upgrade from a version without the fix, flush ASIC_DB at boot in fast-reboot as well.

Related to sonic-net/sonic-utilities#3342

- How I did it
Flush ASIC_DB on fast boot.

- How to verify it
Run fast-reboot.

Signed-off-by: Stepan Blyschak <stepanb@nvidia.com>

files/build_templates/docker_image_ctl.j2

@@ -243,6 +243,9 @@ function postStartAction()
             fi
 
             if [[ "$BOOT_TYPE" == "fast" ]]; then
+                # Flush ASIC DB. On fast-boot there should be nothing in there.
+                # In the older versions there has been an issue where a queued FDB event might get into ASIC_DB causing syncd crash at boot.
+                $SONIC_DB_CLI ASIC_DB FLUSHDB
                 # this is the case when base OS version does not support fast-reboot with reconciliation logic (dump.rdb is absent)
                 # In this case, we need to set the flag to indicate fast-reboot is in progress. Set the key to expire in 3 minutes
                 $SONIC_DB_CLI STATE_DB SET "FAST_REBOOT|system" "1" "EX" "180"