add sample docker structure tests

tests/README-tests.md

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+# Docker Image Structural Testing
+The configuration file `test/config.yaml` defines various "structural" checks to perform on the docker image created by `../Dockerfile`.
+
+The test require the use of the [container-structure-test](https://github.com/GoogleContainerTools/container-structure-test) from Google.
+
+Follow their documentation to install.
+
+## Usage
+Generate your docker image using your preferred method e.g.
+```shell
+docker build -t myimage .
+```
+
+To run the tests against the image, run the following:
+```shell
+container-structure-test test --image myimage --config tests/config.yaml
+```

tests/config.yaml

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+schemaVersion: 2.0.0
+
+commandTests:
+  - name: "say hello world"
+    command: "bash"
+    args:
+      - -c
+      - |
+        echo hello &&
+        echo world