Flush stdout/stderr to avoid empty log in detached docker when start block

[chenk008]

Why are these changes needed?

This change flushes stdout/stderr, so that the detached docker container can show the log.

Without this change:

With this change:

Related issue number

Closes #19545

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[DmitriGekhtman]

Thanks, looks good.
CI looks unhappy -- would you mind rebasing or merging master and retrying?

[chenk008]

@DmitriGekhtman Thanks. I have merged master, I think windows-msvc failure is not related to this PR.