How to use Build Number inside Pipeline step as Image:tag or version?

I want to use Buildkite Build Number inside Pipeline step.
I tried using $BUILDKITE_BUILD_NUMBER inside pipeline step command.
However it gives error on save, saying BUILDKITE_BUILD_NUMBER is not available.
How can I use Build Number inside the pipeline?

Normally in Jenkins, we use Build or Job number inside pipeline while building Image for build tag.