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

Question

I am deploying a .Net Core application using Docker Containers.

I was able to push the image successfully using heroku container:push web

But calling heroku container:release web returns No command specified for process type web.

I'm a bit stumped I've tried adding heroku.yml

setup:
  addons:
    - plan: heroku-postgresql
      as: DATABASE
build:
  docker:
    web: Dockerfile
  config:
    ASPNETCORE_ENVIRONMENT: development
run:
  web: dotnet Api.dll

On the root directory but still no luck.

I can confirm that the image was created I'm seeing this in docker

REPOSITORY                                     TAG                 IMAGE ID            CREATED             SIZE
registry.heroku.com/immense-temple-11020/web   latest              5cb62a9af317        16 minutes ago      272MB

If anyone could point me to the right direction, it would be much appreciated. Thanks!

Answer 1

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."