Delete the apt-get lists after installing something.
RUN apt-get update && apt-get install -y python
RUN apt-get update && apt-get install -y python \ && apt-get clean \ && rm -rf /var/lib/apt/lists/*
In addition, cleaning up the apt cache and removing /var/lib/apt/lists helps keep the image size down. Since the RUN statement starts with apt-get update, the package cache will always be refreshed prior to apt-get install.
Clean up must be performed in the same RUN step, otherwise it will not affect image size.