upgrade actions/checkout to v4

[k-yamasaki-zakisan]

Description:
Thanks for your work. The actions/checkout has been released to v4, I have updated README to use it.

DCO check

[crazy-max]

Thank you for contributing! It appears your commit message is missing a DCO sign-off, causing the DCO check to fail: https://github.com/docker/build-push-action/pull/1041/checks?check_run_id=20888520981

We require all commit messages to have a Signed-off-by line with your name and e-mail, which looks something like:

Signed-off-by: YourFirsName YourLastName <[email protected]>

There is no need to open a new pull request, but to fix this (and make CI pass), you need to amend the commit(s) in this pull request, and "force push" the amended commit.

Unfortunately, it's not possible to do so through GitHub's web UI, so this needs to be done through the git commandline.

You can find some instructions in the output of the DCO check (which can be found in the "checks" tab on this pull request), as well as in the Moby contributing guide.

Steps to do so "roughly" come down to:

1. Set your name and e-mail in git's configuration:

   git config --global user.name "YourFirstName YourLastName"
   git config --global user.email "[email protected]"
   

   (Make sure to use your real name (not your GitHub username/handle) and e-mail)

2. Clone your fork locally

3. Check out the branch associated with this pull request

4. Sign-off and amend the existing commit(s)

   git commit --amend --no-edit --signoff
   

   If your pull request contains multiple commits, either squash the commits (if needed) or sign-off each individual commit.

5. Force push your branch to GitHub (using the --force or --force-with-lease flags) to update the pull request.

Sorry for the hassle (I wish GitHub would make this a bit easier to do), and let me know if you need help or more detailed instructions!

[k-yamasaki-zakisan]

Thanks for the manual. With your help, I was able to configure the DCO. I hope this setup works out well...
if my configuration is wrong, let me know.👋

[crazy-max]

Thanks LGTM!