It may have cached the failure, but let’s test one thing at a time.
You shouldn’t need to add the environment variable, but you can check to see whether adding the certificate worked by doing, for example:
root:~# /var/discourse/launcher enter app
root@app:/var/www/discourse# rails console
[1] pry(main)> Net::HTTP.get URI 'https://meta.discourse.org/about.json'
If you get a result (and you didn’t before), that means the certificate is properly installed. Then you can add commands to the container definition to install that certificate on every rebuild so it’ll persist.