Add cleanup of apt cache and /tmp files in each layer.

This saves ~50Mb for vyos-build image: 2.04Gb -> 1.99Gb
And ~19Mb for vyos image: 155Mb -> 136Mb

Docker stores all files created in each layer so command

RUN wget -O /tmp/open-vmdk-master.zip https://github.com/.../master.zip && \
     unzip -d /tmp/ /tmp/open-vmdk-master.zip && \
     cd /tmp/open-vmdk-master/ && make && make install

will store open-vmdk-master.zip and /tmp/open-vmdk-master
in the image even though there is a cleanup command later:

RUN rm -rf /tmp/*

The cleanup command just makes these files invisible in last layer.

So temporary file must be removed in same RUN command
not to be stored in the image.