As you noticed, it is not installed. You can open a shell in the container and install it manually:
cd /opt/shinobi
npm install discord.js
node tools/modifyConfiguration.js addToConfig='{"discordBot":true}'
Then you copy your conf.json and package.json from /opt/shinobi to /config.
Then docker script copies everything from your mounted config folder into the shinobi folder. That way discord will be installed when you restart the container. When you update the container you should check if package.json got an update