How to avoid building twice in a Jenkinsfile in order to have different image names?

Question

In our project, we append the branch name when we build, unless it's master. And we tag them with the build_id and "latest". This is what we want.

- myapp:latest
- myapp:1
- myapp:2
- myapp:3
- myapp-branch-x:latest
- myapp-branch-x:1
- myapp-branch-x:2
- myapp-branch-x:3
- myapp-branch-y:latest
- myapp-branch-y:1
- myapp-branch-y:2
- myapp-branch-y:3

In order to achieve this, we are building twice when it's on the master branc. I think it's weird. How can we avoid that?

pipeline {
  environment {
    dockerCredentials = 'myapp-dockerhub'
    dockerRegistryUrl = 'https://dockerhub.example.com'
    imageName = "dockerhub.example.com/myapp/myapp"
    build_id = "${BUILD_ID}"
    branch_name = "${BRANCH_NAME.toLowerCase().replace('-$', '')}"
    app = ''
  }
  agent any
  stages {
    stage('Build') {
      steps {
        script {
          app = docker.build(imageName + '-' + branch_name)
          withDockerRegistry(credentialsId: dockerCredentials, url: dockerRegistryUrl) {
            app.push(build_id)
            app.push('latest')
          }
        }
      }
    }
    stage('Build-Master') {
      when {
        branch 'master'
      }
      steps {
        script {
          app = docker.build(imageName)
          withDockerRegistry(credentialsId: dockerCredentials, url: dockerRegistryUrl) {
            app.push(build_id)
            app.push('latest')
          }
        }
      }
    }
  }
}

Answer 1

Currently the first stage executes for all situations, and the second stage executes only on the master branch according to the when expression when { branch 'master' }. You can add a when expression to the first stage to only build non-master branches. Then the master branch will not execute for both stages:

stage('Build') {
  when { not { branch 'master' } }
  ...
}

You can check the when expression documentation for more information.