Customizing Playground for self-hosting with a private theme

I'm trying to create a customized WordPress playground for a private theme that I can host in Github Pages. Following the docs I've been able to reliable set up a self-hosted Playground but I'm unable to get any customization to work.

Before I run a build I customize the Dockerfile, above the block that strips whitespace:

COPY ./build-assets/private-theme.zip wordpress/wp-content/themes/

RUN cd wordpress/wp-content/themes/ && \
    unzip private-theme.zip
    rm private-theme.zip

RUN cd wordpress/wp-content/mu-plugins && \
    # Install plugins
    for plugin_name in gutenberg accordion-blocks breadcrumb-navxt; do \
      curl -L /{$plugin_name}.latest-stable.zip -o {$plugin_name}.zip && \
      unzip $plugin_file && \
      rm $plugin_file && \
      # Create entry file in mu-plugins root
      echo "<?php require_once __DIR__.'/$plugin_name/$plugin_name.php';" > $plugin_name.php; \
    done;

But I don't seem to be able to affect anything. Any recommendations would be appreciated.