Help with docker calls in Groovy

[Chris Fouts]

 I days old with Groovy so please bear with me. I bought a Groovy book but
this subject is of course not discussed in it.

We use a Jenkins file to set up stages, and we have this stage

def demoImage
stage("Build ECS Docker Image") {

         dir('demo') {

            docker.withRegistry("https://${dockerUrl}";, ecrCredentialsId) {

              demoImage = docker.build("${dockerUrl}/some-name-
${env.EnvironmentKey}-${env.DBName}")

            }

          }
}

I assume demoImage contains some image information, like the image ID that
I need for later? How do I get the image ID from demoImage? Where would
this be documented online? I googled, but I don't necessarily how to look
what I'm looking for.

Thanks,
Chris