[RFC] Stop building Docker images in Pull Requests

[crcrpar]

Motivation

Docker build jobs on Pull Request pushes sometimes push down the CI job results which are more critical and required such as tests-python37.
Also, I think we can reckon that Docker build works if CI for Pull Requests pass as these two do almost exactly the same in the installation phase.

Description of the changes

• Change the trigger of this job
• Remove unused environment variable
• Remove if statement

[HideakiImamura]

Thanks for the PR. I agree with you. Certainly, there is no need to build many docker images every time per push to a PR. However, there may be some developers using the generated images, so I'm concerned that this might be an unfriendly change. Don't we need to investigate how the generated images are being used by developers?

[crcrpar]

First of all, much appreciate your comment.

However, there may be some developers using the generated images, so I'm concerned that this might be an unfriendly change. Don't we need to investigate how the generated images are being used by developers?

As the workflow shows, currently, there is almost no way to pull docker images built for PRs' commits since we only push the docker images on push to master. So, I don't think we have to do it.

[HideakiImamura]

Thanks for the confirmation! If we don't use the created docker images at all, we don't have to create many images in PRs. LGTM for these changes.

[toshihikoyanase]

Thank you for your PR!
I'm a bit concerned about the review of Dockerfile and setup.py. Is it possible to run the docker build workflow if the PR contains the changes regarding Dockerfile and setup.py?

[crcrpar]

I'm a bit concerned about the review of Dockerfile and setup.py. Is it possible to run the docker build workflow if the PR contains the changes regarding Dockerfile and setup.py?

It should be possible per https://help.github.com/en/actions/reference/workflow-syntax-for-github-actions#example-including-paths and gave it a try in 3ce0409.

[toshihikoyanase]

LGTM. Thank you for your update!

[toshihikoyanase]

[Note] The trigger is not limited to the master branch, but I think it is acceptable because the built images are not uploaded to the DockerHub.