.github/workflows/compilers.yml: Specify a container running user as root.

[junaruga]

Coming new ruby/ruby-ci-image images are required to run the container as
a regular user by default, while the root user is required to run the
compilers.yml. Add id command to print the user info.

Co-authored-by: fedor fedor@cirruslabs.org

---

This PR is based on the PR: #4750 . I updated the commit message, and container image URL and added the id command. This PR is a preparation for the coming new images.

Note the new ruby/ruby-ci-image images are here.
ruby/ruby-ci-image#3

I tested this PR with both the current and coming container images too.

The newly added id command and current mkdir build command only work with working-directory: syntax. But doesn't work without the working-directory: syntax.

       - run: id
         working-directory:
       - run: mkdir build
         working-directory:

For example, the mkdir build without the working-directory: syntax prints the following error. I am not sure why the error happen. Anyway, I added the working-directory: to the commands.

https://github.com/junaruga/ruby/runs/3404442652?check_suite_focus=true#step:4:17

Run mkdir build
OCI runtime exec failed: exec failed: container_linux.go:380: starting container process caused: chdir to cwd ("/__w/ruby/ruby/build") set in config.json failed: no such file or directory: unknown
Error: Process completed with exit code 126.

[nobu]

As the working-directory is set to build at the bottom of that file, any command can’t run without resetting it before the mkdir has done.

[junaruga]

Ah I understood it now seeing the following bottom line of the file. Thanks for the explanation. When a command is executed without working-directory: before creating the build directory, the command is executed on the build directory but the build directory is still not created. So, we cannot run the command.

defaults:
  run:
    working-directory: build