Add script and instruction to build all the docker images by yourself

contrib/DockerFileBuildHelper/README.md

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+# DockerFile build helper
+
+By default, when you use docker deployment, you are fetching pre-built docker images hosted on dockerhub.
+While this bring the advantage that deployment is fast and reliable, this also mean that you are ultimately trusting the owner of the docker images.
+This repository generate a script that you can use to build all images from the sources by yourself.
+
+## How to use?
+
+Install [.NET Core SDK](https://dotnet.microsoft.com/download) and run:
+
+```bash
+./run.sh
+```
+
+This will build a `build-all.sh` file.
+Note that the