Add testing of C++ with sanitizers (CI-Docker) (#5631)

* Add C++ build testing with clang and gcc

This adds Dockerfiles which test building flatc and the C++ library against clang
and gcc. See discussion at #5119.  It is derived from the Travis CI tooling.

The GRPC tests are failing due to #5099 so those are commented out.

These are run from the .travis.yml file rather than the tests/docker/languages
folder because the builds may each take longer than 30 minutes and were hitting
Travis timeouts.

Parallel builds and build caching attempt to keep the build times low.

* Add GCC 8.3 and Clang 7.0 with sanitizers into CI (based on #5130)

- Add a docker based on Debian Buster.
- Add C++ building scripts for the docker.
- Leak-sanitizer requires SYS_PTRACE.

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

@@ -15,10 +15,15 @@
 # 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
+docker build -t build_cpp_image -f tests/docker/Dockerfile.testing.cpp.debian_buster .
+# Run tests with sanitizers  (--cap-add SYS_PTRACE), both GCC and Clang.
+cpp_test_args="--cap-add SYS_PTRACE build_cpp_image sh ./tests/docker/cpp_test.run.sh Debug"
+docker run --rm $cpp_test_args
+docker run --rm --env CC=/usr/bin/clang --env CXX=/usr/bin/clang++ $cpp_test_args
+# Build flatc on debian once to speed up the test loop below.
+docker run --name flatc_container build_cpp_image sh ./tests/docker/build_flatc.run.sh Debug
+# All dependent dockers refer to 'flatc_debian_stretch'.
+docker cp flatc_container:/flatbuffers/flatc flatc_debian_stretch
 
 for f in $(ls tests/docker/languages | sort)
 do