Adds Kokoro configuration (GoogleCloudPlatform#577)

Author: bshaffer

.kokoro/common.cfg

@@ -0,0 +1,13 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Download trampoline resources. These will be in ${KOKORO_GFILE_DIR}
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/trampoline"
+
+# All builds use the trampoline script to run in docker.
+build_file: "php-docs-samples/.kokoro/trampoline.sh"
+
+# Configure the docker image for kokoro-trampoline.
+env_vars: {
+    key: "TRAMPOLINE_IMAGE"
+    value: "gcr.io/cloud-devrel-kokoro-resources/php"
+}

.kokoro/php72/docker/Dockerfile

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+FROM gcr.io/google_appengine/php72
+
+RUN composer global require phpunit/phpunit:4.8
+
+RUN export PATH="$PATH:/opt/composer/vendor/bin"
+
+ENTRYPOINT /bin/bash

.kokoro/system_tests.cfg

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+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Download secrets from Cloud Storage.
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/php-docs-samples"
+
+# Tell the trampoline which build file to use.
+env_vars: {
+    key: "TRAMPOLINE_BUILD_FILE"
+    value: "github/php-docs-samples/.kokoro/system_tests.sh"
+}

.kokoro/system_tests.sh

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+#!/bin/bash
+echo "hello world"

.kokoro/trampoline.sh

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+#!/bin/bash
+
+# Copyright 2017 Google Inc.
+#
+# 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.
+
+python3 "${KOKORO_GFILE_DIR}/trampoline_v1.py"