General help

You may be misunderstanding how docker volumes work. Volumes are a persistent directories on the host that are mounted into a container at runtime. You cannot write into a volume from your dockerfile, at build time. I don't know exactly what will happen if your dockerfile does write to a location where you then mount a volume (if this is even possible), but my guess is that the container will see the contents of the volume, and not the files written at build time.

In the example I shared above in your dockerfile, docker will download plugin.zip to /, then unzip its contents into /path/to/plugindir, which should be the path to that plugin type's directory within your moodle codebase. You will already need to have copied your moodle codebase to this location, earlier in the dockerfile. This will load all of the PHP into the container's filesystem, meaning it will exist as soon as the container starts up without having to mount a volume from the host, and any changes to those files will be lost when the container shuts down. If you need to update the files, you will need to rebuild the container and redeploy it.

An alternative approach using volumes would be to manage all your moodle codebase in a directory on the host, then mount that directory onto your container at runtime. This will mean any changes made to the code are reflected "live".

The former approach is more appropriate for production deployment, while the latter is better suited to a development environment.