I'm running a jenkins instance on GCE inside a docker container and would like to execute a multibranch pipeline from this Jenkinsfile and Github. I'm using the GCE jenkins tutorial for this. Here is my Jenkinsfile

node {
  def project = 'xxxxxx'
  def appName = 'gceme'
  def feSvcName = "${appName}-frontend"
  def imageTag = "eu.gcr.io/${project}/${appName}:${env.BRANCH_NAME}.${env.BUILD_NUMBER}"

  checkout scm

  sh("echo Build image")
  stage 'Build image'
  sh("docker build -t ${imageTag} .")

  sh("echo Run Go tests")
  stage 'Run Go tests'
  sh("docker run ${imageTag} go test")

  sh("echo Push image to registry")
  stage 'Push image to registry'
  sh("gcloud docker push ${imageTag}")

  sh("echo Deploy Application")
  stage "Deploy Application"
  switch (env.BRANCH_NAME) {
    // Roll out to canary environment
    case "canary":
        // Change deployed image in canary to the one we just built
        sh("sed -i.bak 's#eu.gcr.io/cloud-solutions-images/gceme:1.0.0#${imageTag}#' ./k8s/canary/*.yaml")
        sh("kubectl --namespace=production apply -f k8s/services/")
        sh("kubectl --namespace=production apply -f k8s/canary/")
        sh("echo http://`kubectl --namespace=production get service/${feSvcName} --output=json | jq -r '.status.loadBalancer.ingress[0].ip'` > ${feSvcName}")
        break

    // Roll out to production
    case "master":
        // Change deployed image in canary to the one we just built
        sh("sed -i.bak 's#eu.gcr.io/cloud-solutions-images/gceme:1.0.0#${imageTag}#' ./k8s/production/*.yaml")
        sh("kubectl --namespace=production apply -f k8s/services/")
        sh("kubectl --namespace=production apply -f k8s/production/")
        sh("echo http://`kubectl --namespace=production get service/${feSvcName} --output=json | jq -r '.status.loadBalancer.ingress[0].ip'` > ${feSvcName}")
        break

    // Roll out a dev environment
    default:
        // Create namespace if it doesn't exist
        sh("kubectl get ns ${env.BRANCH_NAME} || kubectl create ns ${env.BRANCH_NAME}")
        // Don't use public load balancing for development branches
        sh("sed -i.bak 's#LoadBalancer#ClusterIP#' ./k8s/services/frontend.yaml")
        sh("sed -i.bak 's#eu.gcr.io/cloud-solutions-images/gceme:1.0.0#${imageTag}#' ./k8s/dev/*.yaml")
        sh("kubectl --namespace=${env.BRANCH_NAME} apply -f k8s/services/")
        sh("kubectl --namespace=${env.BRANCH_NAME} apply -f k8s/dev/")
        echo 'To access your environment run `kubectl proxy`'
        echo "Then access your service via http://localhost:8001/api/v1/proxy/namespaces/${env.BRANCH_NAME}/services/${feSvcName}:80/"
  }
}

I always get an error docker not found:

[apiservice_master-GJCRJX6ZJPDVVSEUHIS6VBX7OYMFS5WKRVRKCSF4PSO76ZGZPKFQ] Running shell script
+ docker build -t eu.gcr.io/xxxxx/apiservice:master.1 .
/var/jenkins_home/workspace/apiservice_master-GJCRJX6ZJPDVVSEUHIS6VBX7OYMFS5WKRVRKCSF4PSO76ZGZPKFQ@tmp/durable-b4503ecc/script.sh: 2: /var/jenkins_home/workspace/apiservice_master-GJCRJX6ZJPDVVSEUHIS6VBX7OYMFS5WKRVRKCSF4PSO76ZGZPKFQ@tmp/durable-b4503ecc/script.sh: docker: not found

What do I have to change to make docker work inside jenkins?