docker: Increase max packet size to 128M

I've seen "MySQL server has gone away" errors in the logs, and this time
it isn't the timeout; one of the suggested fixes is to increase the max
packet size (and I have also seen errors in the mariadb logs indicating
that the max has been exceeded).

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

docker-compose.yml

@@ -2,7 +2,7 @@ version: '3'
 services:
   layersdb:
     image: mariadb:10.2
-    command: --character-set-server=utf8mb4 --collation-server=utf8mb4_unicode_ci --wait_timeout=28800
+    command: --character-set-server=utf8mb4 --collation-server=utf8mb4_unicode_ci --wait_timeout=28800 --max_allowed_packet=128M
     environment:
      - "MYSQL_DATABASE=layersdb"
      - "MYSQL_ROOT_PASSWORD=testingpw"