Squashed 'third_party/flatbuffers/' content from commit acc9990ab

Change-Id: I48550d40d78fea996ebe74e9723a5d1f910de491
git-subtree-dir: third_party/flatbuffers
git-subtree-split: acc9990abd2206491480291b0f85f925110102ea
diff --git a/tests/docker/languages/Dockerfile.testing.csharp.mono_5_18 b/tests/docker/languages/Dockerfile.testing.csharp.mono_5_18
new file mode 100644
index 0000000..e6ba550
--- /dev/null
+++ b/tests/docker/languages/Dockerfile.testing.csharp.mono_5_18
@@ -0,0 +1,8 @@
+FROM mono:5.18 as base
+WORKDIR /code
+ADD . .
+RUN cp flatc_debian_stretch flatc
+WORKDIR /code/tests
+RUN mono --version
+WORKDIR /code/tests/FlatBuffers.Test
+RUN sh NetTest.sh