Customizing Playground for self-hosting

I’m going to start with the support forums before moving to other venues.

I’m trying to build a custom self-hosted Playground (via GitHub Pages) with a private theme. Following the docs I can reliably get a Playground up and running but I seem unable to affect the WordPress install.

In the Dockerfile above the block to strip whitespace, to slip in some plugins, I’ve added:

RUN cd wordpress/wp-content/mu-plugins && \
    # Install plugins
    for plugin_name in gutenberg accordion-blocks breadcrumb-navxt; do \
      curl -L https://downloads.www.ads-software.com/plugin/{$plugin_name}.latest-stable.zip -o {$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;

And to add a theme:

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

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

And I’m not getting anything custom after running the build. I’m not sure if I’m fundamentally misunderstanding how this process should work or not. Any help would be appreciated.