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.

You can read more about this here.

Note: Clean up must be performed in the same RUN step, otherwise it will not affect image size.

Bad Practice

RUN apt-get update && apt-get install -y python

Recommended

RUN apt-get update && apt-get install -y python \
 && apt-get clean \
 && rm -rf /var/lib/apt/lists/*

Exceptions

The official Debian and Ubuntu images are configured to automatically run apt clean, so explicitly invoking apt clean is not necessary there.

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No ignore rules found