CI: Dockerized language port tests (#5066)

This runs a script in TravisCI that executes a bunch of small Docker image
scripts to test the language ports in isolated environments. This allows us to
test multiple language versions with little additional complexity.

Covers:

+ Java OpenJDK 10.0.2
+ Java OpenJDK 11.0.1
+ Node 10.13.0
+ Node 11.2.0
+ Python CPython 2.7.15
+ Python CPython 3.7.1
+ Rust 1.30.1

.travis/build-and-run-docker-test-containers.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+#
+# 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.
+set -e
+
+# build flatc on debian once to speed up the test loop below
+docker build -t build_flatc_debian_stretch -f tests/docker/Dockerfile.testing.build_flatc_debian_stretch .
+BUILD_CONTAINER_ID=$(docker create --read-only build_flatc_debian_stretch)
+docker cp ${BUILD_CONTAINER_ID}:/code/flatc flatc_debian_stretch
+
+for f in $(ls tests/docker/languages | sort)
+do
+        # docker pull sometimes fails for unknown reasons, probably travisci-related. this retries the pull we need a few times.
+        REQUIRED_BASE_IMAGE=$(cat tests/docker/languages/${f} | head -n 1  | awk ' { print $2 } ')
+
+        set +e
+        n=0
+        until [ $n -ge 5 ]
+        do
+           docker pull $REQUIRED_BASE_IMAGE && break
+           n=$[$n+1]
+           sleep 1
+        done
+        set -e
+
+        docker build -t $(echo ${f} | cut -f 3- -d .) -f tests/docker/languages/${f} .
+        echo "TEST OK: ${f}"
+done