Append new source to sources.list, don't overwrite.

If you overwrite the sources.list then the apt-get won't be able to install it
apt-get install -y openjdk-7-jre-headless unzip

It will fail with:

    Some packages could not be installed. This may mean that you have
    requested an impossible situation or if you are using the unstable 
    distribution that some required packages have not yet been created
    or been moved out of Incoming.
    The following information may help to resolve the situation:

    The following packages have unmet dependencies:
     openjdk-7-jre-headless : Depends: tzdata-java but it is not going to be installed
    E: Unable to correct problems, you have held broken packages.