blender customization example in doc (#378)

Add blender dockerfile example to customize editor image and support .blend

docs/08-docker/03-customize-docker-images.mdx

@@ -110,3 +110,34 @@ customize the docker image used during build using
 
 For more information on how to publish to docker hub, you may refer to
 [docker's documentation](https://docs.docker.com/docker-hub/repos/)
+
+## Example : Include Blender
+
+To include blender in the image used to build your unity project, you can define a custom Dockerfile
+based on unityci/editor and install blender on top of it.
+
+### Dockerfile
+
+```dockerfile
+ARG GAMECI_IMAGE = unityci/editor:2021.3.1f1-windows-mono-1.0.1
+FROM $GAMECI_IMAGE
+
+ARG BLENDER_SHORT_VERSION=3.4
+ARG BLENDER_FULL_VERSION=3.4.1
+
+RUN echo "BLENDER_SHORT_VERSION: $BLENDER_SHORT_VERSION"
+RUN echo "BLENDER_FULL_VERSION: $BLENDER_FULL_VERSION"
+RUN apt-get update && \
+    apt-get install -y wget && \
+    wget https://download.blender.org/release/Blender$BLENDER_SHORT_VERSION/blender-$BLENDER_FULL_VERSION-linux-x64.tar.xz && \
+    tar -xf blender-$BLENDER_FULL_VERSION-linux-x64.tar.xz && \
+    rm blender-$BLENDER_FULL_VERSION-linux-x64.tar.xz
+
+ENV PATH="$PATH:/blender-$BLENDER_FULL_VERSION-linux-x64"
+
+```
+
+### Usage
+
+You can build this image and upload it to your own Dockedhub registry to reference it in your Unity
+pipeline afterward.