How do I use Docker's --cache-from build flag in a declarative Jenkins pipeline?

Question

I'm using a declarative Jenkinsfile to run some stages inside a Docker container. The process works okay but the build times are often very slow as our CI has quite a few slaves and if the build occurs on a slave without the layer cache, the entire build takes a while.

I've read that Docker can speed up builds if a --cache-from flag is specified. How do I specify the cache-from flag and the external registry's URL and credentials?

pipeline {

  agent { dockerfile true }
  environment {
    REPO = credentials('supersecret')
  }

  stages {
    stage('Prepare environment') {
      steps {

Answer 1

The pipeline syntax do authorize additional parameters

You can pass additional arguments to the docker build ... command with the additionalBuildArgs option, like agent

{ dockerfile { additionalBuildArgs '--build-arg foo=bar' } }

But cache-from refer to an image which might be in a dedicated external registry with its own credentials.
Maybe you can setup an first step just in charge of docker login in that registry.

---

Another approach entirely would be to reuse the same node for that specific build.
See "Reusing node/workspace with per-stage Docker agents"

pipeline {
  agent {
    label 'whatever'
  }
  stages {
    stage('build') {
      steps {
        sh "./build-artifact.sh"
      }
    }
    stage('test in docker') {
      agent {
        docker {
          image 'ubuntu:16.04'
          reuseNode true
        }
      }
      steps {
        sh "./run-tests-in-docker.sh"
      }
    }
  }
}

Then any docker build would benefit from the current local image cache.