Heroku CLI "No command specified for process type web" when releasing container

Did you have a CMD directive specified at the end of your Dockerfile? If not, that was likely causing the problem, which you then worked around by using heroku.yml and heroku stack:set container.

Heroku is unclear on this in their documentation. I was able to get my container to run locally without a CMD directive, likely as you were. But if you read Heroku docs here or here you'll pick up that Heroku takes for granted that the Dockerfile will specify a CMD to run.

I had the same problem until I added the CMD directive at the end of my Dockerfile. I'm still working though what exactly I should put as my CMD directive but I no longer get the No command specified for process type web error.

Since you switched to using heroku.yml you were able to get around the CMD by using

run:
  web: dotnet Api.dll

Heroku docs say "If you don’t include a run section, Heroku uses the CMD specified in the Dockerfile."