Symlink command in Dockerfile doesn't create the link in the container

Question

In my Dockerfile I have this line:

RUN ln -s /var/www/html/some_file /var/www/html/another_file

When running docker build all the steps are executed including the creation of the symbolic link, but when I start a container using the image created and check the folder /var/www/html/ I don't see the link there. I tried searching online if this is something supported by docker and couldn't find an answer. The content of the container is already available by another container image I am using with the FROM instruction, so the file /var/www/html/some_file is not on my machine.

No Volumes are involved. This is the Dockerfile:

FROM piwik:3.2.1-apache

RUN apt update

RUN ln -s /var/www/html/some_file /var/www/html/another_file

CMD [ "apache2-foreground" ]

Answer 1

The Dockerfile for that image (three and a half years old and not getting any updates!) has the line:

VOLUME /var/www/html

This will prevent any subsequent RUN instructions from making any changes to that directory, even if it's in a derived image.

There's no way to un-VOLUME a directory, so if you need this symlink to exist, you either need to create a new image from a base PHP image installing the software yourself, or wrap its entrypoint script with your own that creates the symlink at startup time.