I think this dockerfile is missing some steps. Upon starting an interactive session in the container, I see the twee2 command doesn't exist. I had to first run:

bundle install
gem build twee2.gemspec
gem install ./twee2-0.5.0.gem

But then when I tried running twee2 I received the error:

You must use Bundler 2 or greater with this lockfile.

It seems there were multiple versions of bundler installed (the one in Gemfile.lock was v2.1, the one in the .gemspec was v1.6) so I had to delete the lockfile.

All that stuff should be handled in the dockerfile, and I believe the twee2 command should be set as the ENTRYPOINT