automatically set existing DATABASE_URL for redeploy of linked app

[srt32]

Upon redeploying an app (having previously created a db and linking the existing app to the db) the redeployed app does not have access to the previously set DATABASE_URL. This makes sense because the new container won't have the env vars so my question is what is the preferred way to programmatically set the DATABASE_URL env var for a frequently deployed app?
#23 seems promising but I still can't quite figure out what's going on. Also, this issue from docker seems promising but doesn't appear to automatically set env vars: moby/moby#240