For unknown reasons to me when you try to run the Dockerfile containing this line, it will not work:

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RUN echo "deb http://http.debian.net/debian jessie-backports main" >> /etc/apt/sources.list

After the machine is booted however you can insert the line succesfully. You can solve this by running the command below:

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RUN echo "deb http://http.debian.net/debian jessie-backports main" > /etc/apt/sources.list.d/backports.list

This creates a new file in the sources.list.d and it gets picked up by apt-get update. Make sure the extension is .list.