Replace Declarative Docker agent directive with new implementation

Goals

• We want to have a new implementation for running steps inside a Docker container that is transparent to the Pipeline author
• We want to ensure that as this developed that we start making changes that allow us to swap out the implementation in the future (e.g. docker, k8, etc)
• We want customers to try this new implementation without having to use patched plugins or special builds from a branch

Scope

• Declarative Docker agent directive can be used to run steps for a stage or for the whole pipeline
• The new implementation can be enabled by specifying Pipeline->Options->dockerPipelineVersion = 2
• Must be able to use an off the shelf docker container without modifying it so it works with Jenkins
• Honour the entry point of the container
• Sharing the workspace from the top level agent to a per stage Docker agent (and back again!)

Out of scope

• Declarative Docker agent directive that specifies a Dockerfile
• Scripted docker.inside
• Scripted docker.build
• Building containers
• Linked containers
• Caching directories

Acceptance criteria
The following steps should work using the new implementation (even if modifications are needed to these steps):

• withMaven()
• dir()
• withCredentials

The following functionality should work:

• Tool installers
• Config File Provider

Examples
Per stage

pipeline {
  options {
    dockerPipelineVersion(2)
  }
  agent none // Will not reuse workspace
  stages {
     stage (‘build’) {
        agent { docker ‘maven3’ } 
        steps { 
	   sh ‘mvn clean test’
        } 
     }
  }
}

Whole Pipeline

pipeline {
  options {
    dockerPipelineVersion(2)
  }
  agent { docker ‘maven3’ } // will reuse workspace for all stages unless specified
  stages {
     stage (‘build’) {
        steps { 
	   sh ‘mvn clean test’
        } 
     }
  }
}