Add example build_mount_cache_apt

[ondrejsika]

FROM ubuntu
RUN \
  rm -f /etc/apt/apt.conf.d/docker-clean && \
  echo 'Binary::apt::APT::Keep-Downloaded-Packages "true";' > /etc/apt/apt.conf.d/keep-cache
ARG PACKAGES="vim"
RUN \
  --mount=type=cache,target=/var/cache/apt \
  --mount=type=cache,target=/var/lib/apt \
  apt update && \
  apt-get install -y --no-install-recommends \
    $PACKAGES