Why doesn't Docker cache all commands?

Question

Docker

Goodnight.
Usually, most commands when rebuilding an image use the cache and are executed instantly. However, some (longest ones) are not cached and are stubbornly executed again from scratch. For example, here it is:

# Gems and bundle
RUN cd /app/ \
    && bundle config git.allow_insecure true \
    && bundle config build.nokogiri --use-system-libraries \
    && bundle install --without development test \
    && apt-get purge -y \
      git \
      openssh-client \
      make \
      g++ \
      libxml2-dev \
      libxslt1-dev \
      libmagickwand-dev \
      libmariadbclient-dev \
      libsqlite3-dev \
      postgresql-server-dev-9.6 \
      libjpeg-dev \
      libpng-dev \
      libtiff-dev \
      libwebp-dev \
    && rm -rf /var/lib/apt/lists/* \
    && rm -rf /root/.ssh /tmp/* /var/tmp/*

I would really like that when rebuilding, if nothing in the command has changed, Docker also takes the result of executing this command from the cache.
Why are some commands not cached, and how to make sure that they are cached?
Thanks in advance.

Answer 1

Share the weight of the Dockerfile, or just view it yourself. If at some stage there were changes, then all subsequent caches are not used.