BIGTOP-3266: Rat check failed for one file with unapproved/unknown licenses

Add .dockerignore to Rat exclude-file list.

Change-Id: Ia4a052db2df7d1e8e87bcde3304ea401a706759b
Signed-off-by: Yuqi Gu <yuqi.gu@arm.com>
diff --git a/build.gradle b/build.gradle
index 8dc9095..66784eb 100644
--- a/build.gradle
+++ b/build.gradle
@@ -140,6 +140,7 @@
        "provisioner/docker/config/hieradata/PLACEHOLDER",
        "provisioner/docker/config/hosts",
        "bigtop_toolchain/files/*.patch",
+       "bigtop-bigpetstore/bigpetstore-transaction-queue/.dockerignore",
   ]
 }