adding USER command

integration_tests/dockerfiles/Dockerfile_test_user_run

@@ -0,0 +1,19 @@
+# Copyright 2018 Google, Inc. All rights reserved.
+#
+# 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.
+
+FROM gcr.io/google-appengine/debian9
+RUN useradd testuser
+RUN groupadd testgroup
+USER testuser:testgroup
+RUN echo "hey" > /etc/foo

integration_tests/dockerfiles/test_user.yaml

@@ -0,0 +1,15 @@
+schemaVersion: '2.0.0'
+commandTests:
+- name: 'whoami'
+  command: 'whoami'
+  expectedOutput: ['testuser']
+  excludedOutput: ['root']
+- name: 'file owner'
+  command: 'ls'
+  args: ['-l', '/tmp/foo']
+  expectedOutput: ['.*testuser.*', '.*testgroup.*']
+  excludedOutput: ['.*root.*']
+fileContentTests:
+- name: "/tmp/foo"
+  path: "/tmp/foo"
+  expectedContent: ["hey"]