ps:forward (and others) enforce buildpack installed even for container deployes

[brian-mcnamara]

Currently the CLI requires a buildpack (apparently any is fine, the check is for size > 0) for forwarding a port using ps:forward (ps:exec too). This is required for slug deploys however buildpacks have no use for container deploys.

The current issue is:

heroku ps:forward 9090 -a APP_NAME
 ▸    $APP_NAME has no Buildpack URL set. You must deploy your application first!

We do manually setup the exec for the container following https://devcenter.heroku.com/articles/exec#using-with-docker

The fix:

The best path forward may simple be to check if the app is a container image and skip the buildpack check.