You still need to move to an up-to-date OS to have a supported version of docker. You will not be able to upgrade (easily?) without upgrading your OS so that you have a supported docker.
You need to remove that from your app.yml.
There is a chance that if you remove that line from your app.yml, but it’s at least as likely that it’ll make your site unusable. I’d definitely make a backup before you do anything else.