Question: how to refer to docker image built in a previous CI step?

Hello, my automation configuration contains sevaral steps. One of them builds a docker image with custom image:tag (includes branch name). Next step should use that docker image. However, I haven't found a way to pass docker image name to `container( image = ... )`.

I tried env variables and DSL functions/variables, but neither of it have worked.

How can I define container image in runtime?

job("build and test") {
    docker {
        beforeBuildScript {
            // create an env variable BRANCH
            content = """
                export BRANCH=${'$'}(echo ${'$'}JB_SPACE_GIT_BRANCH | cut -d'/' -f 3)
            """
        }
        build {}
        
        push(rootImage) {
            tags ("\$BRANCH")
        }
    }
    // what to place here?
    // neither of following worked:
    // $rootImage:$BRANCH
    // $rootImage:\$BRANCH
    // $rootImage:${'$'}BRANCH
    container(displayName = "run tests", image = "$rootImage:??????????????????")) {   
        shellScript {   
            content = """
                pip install pytest-cov
                pytest --show-capture stderr --cov=. --cov-report term:skip-covered
            """
        }
    }
}