build-sys: Run gofmt -d in 'make check'

Signed-off-by: Stefan Berger <stefan@linux.ibm.com>

Makefile

@@ -37,6 +37,7 @@ bin/ctr-enc: cmd/ctr FORCE
 check:
 	@echo "$@"
 	@golangci-lint run
+	@script/check_format.sh
 
 install:
 	@echo "$@"

script/check_format.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+#   Copyright The containerd Authors.
+
+#   Licensed under the Apache License, Version 2.0 (the "License");
+#   you may not use this file except in compliance with the License.
+#   You may obtain a copy of the License at
+
+#       http://www.apache.org/licenses/LICENSE-2.0
+
+#   Unless required by applicable law or agreed to in writing, software
+#   distributed under the License is distributed on an "AS IS" BASIS,
+#   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#   See the License for the specific language governing permissions and
+#   limitations under the License.
+
+out=$(gofmt -d ./cmd ./images)
+[ $? -ne 0 ] && exit 1
+if [ "$out" != "" ]; then
+	echo "$out"
+	exit 1
+fi
+exit 0