why does the anchor folder not exist in the docker image?

Dockerfile

FROM mprasil/dokuwiki
RUN apt-get update && \
    apt-get install -y unzip curl && \
    apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
RUN curl -O -L "https://github.com/turnermm/ckgedit/archive/master.zip" && \
    unzip master.zip -d /dokuwiki/lib/plugins/ && \
    mv /dokuwiki/lib/plugins/ckgedit-master /dokuwiki/lib/plugins/ckgedit && \
    rm -f master.zip
RUN curl -O -L "https://trello-attachments.s3.amazonaws.com/5af4815352fa15728c62aaae/5afe9c8a66d239ee43a6f068/92c13557957b1d6893779951e94ef181/anchor.zip"
RUN unzip anchor.zip -d /dokuwiki/lib/plugins
RUN ls /dokuwiki/lib/plugins/

log

...
Step 5/7 : RUN unzip anchor.zip -d /dokuwiki/lib/plugins
---> Running in 68a5821c16c0
Archive:  anchor.zip
  creating: /dokuwiki/lib/plugins/anchor/
 inflating: /dokuwiki/lib/plugins/anchor/plugin.info.txt  
 inflating: /dokuwiki/lib/plugins/anchor/syntax.php  
---> e43721c97c84
Removing intermediate container 68a5821c16c0
Step 6/7 : RUN ls /dokuwiki/lib/plugins/
---> Running in d8e82a8aaa1a
acl
action.php
admin.php
auth.php
authad
authldap
authmysql
...
---> 0e2b9eabd164

Answer: the image we inherit from declares the plugin folder as volume. being a volume, the folder is lost after each step. Some more on this at https://container-solutions.com/understanding-volumes-docker/