THRIFT-3907 Skip Docker image build on CI when unchanged

Add missing docker tag command for skip path.
diff --git a/build/docker/check_unmodified.sh b/build/docker/check_unmodified.sh
index 564e4c1..9d5fa26 100755
--- a/build/docker/check_unmodified.sh
+++ b/build/docker/check_unmodified.sh
@@ -35,6 +35,7 @@
 
 if try_pull; then
   echo Dockerfile seems identical. No need to rebuild from scratch.
+  docker tag thrift/thrift-build:$DISTRO thrift-build:$DISTRO
 else
   echo Either Dockerfile has changed or pull failure. Need to build brand new one.
   exit 1