Add CI step to check license headers

Signed-off-by: Guillaume Lours <guillaume.lours@docker.com>

scripts/validate/fileheader

@@ -0,0 +1,28 @@
+#!/usr/bin/env sh
+
+#   Copyright The Compose Specification 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.
+
+
+set -eu -o pipefail
+
+if ! command -v ltag; then
+    >&2 echo "ERROR: ltag not found. Install with:"
+    >&2 echo "    go get -u github.com/kunalkushwaha/ltag"
+    exit 1
+fi
+
+BASEPATH="${1-}"
+
+ltag -t "${BASEPATH}scripts/validate/template" -excludes "validate testdata" --check -v