Flesh out docker-compose Cookiecutter template

docker-compose/examples/grafana/build-context/grafana/Dockerfile

@@ -0,0 +1,14 @@
+# For the remainder of this Dockerfile EXAMPLE_ARG_FOR_DOCKERFILE will be
+# available with a value of 'must_be_available_in_dockerfile', check out the env
+# file at 'env/fully.qualified.domain.name.example' for reference.
+# ARG EXAMPLE_ARG_FOR_DOCKERFILE
+
+# Another env var, this one's needed in the example build step below:
+# ARG GRAFANA_VERSION
+
+# Example
+# FROM "grafana:${GRAFANA_VERSION}"
+# RUN apt-get update && \
+#     apt-get -y install \
+#     somepackage-6.q16-6-extra && \
+#     rm -rf /var/lib/apt/lists/*