即使进行了清理，Dockerfile RUN 也会创建一个胖层

Question

It's generally understood that if the RUN instruction cleans up after large file operations, the resulting layer size would be smaller. However when I execute the following script the resulting layer is large and roughly corresponds to the size of the files initially copied and cleaned.

How could this size of a layer be committed if the files copied are cleaned in the same layer?

RUN mkdir -p /etc/puppet && \
    cd /etc/puppet && \
    apt-get update && apt-get install -y wget puppet && \
    wget -rnH --level=10 -e robots=off --reject "index.html*" http://somefileserver.org/puppet/allpuppetstuff && \
    puppet module install puppetlabs/stdlib && \
    puppet module install 7terminals-java && \
    puppet apply -e "include someserver" && \
    apt-get purge -y --auto-remove puppet wget && \
    rm -rfv /etc/puppet/* && \
    rm -rfv /var/lib/apt/lists/*

Answer 1

Found some details after few tweaks.

It seems docker duplicates on old layer, if a newer layer is modifying files persisted on the earlier one. This is the cause of the fat layer created by the above RUN command, even when the total file size diff at the end of operation is minimal. I couldn't find any definite resources to cite this, but this is the experience so far.

Basically, file operations related to the image should always be done in one layer.