gistfile2.txt

// edited for brevity (removed withCredentials etc)
node {
  stage "Build"
  milestone()

  checkout scm
  sh "docker build -t mybuild-${env.BUILD_ID}-image ."
  def test_node_spec = "\"${NODE_NAME}\""

  def branches = [:]
  for (int i = 0; i < 2; i++) {
    def index = i
    branches["Shard "+index] = {
      node(test_node_spec) {
        sh "docker run mybuild-${env.BUILD_ID}-image"
      }
    }
  }
  stage "Test"
  parallel branches
}

gistfile3.txt

bash-4.3# ps auxf
PID   USER     TIME   COMMAND
    1 root       5:11 java -Dorg.jenkinsci.remoting.engine.JnlpProtocol3.disabled=true -c
 1132 root       0:00 [docker]
 1190 root       0:00 [docker]
 4299 root       0:00 bash
 4305 root       0:00 ps auxf

thread-dump.txt

Thread #42
	at DSL.sh(completed process (code -1) in /var/jenkins_home/workspace/<snip>@3@tmp/durable-068ac43e on JNLP4-connect connection from <snip> (pid: 5007); recurrence period: 0ms)
	at WorkflowScript.run(WorkflowScript:35)
	at DSL.withCredentials(Native Method)
	at WorkflowScript.run(WorkflowScript:34)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:28)
	at DSL.parallel(Native Method)
	at WorkflowScript.run(WorkflowScript:59)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:3)
Thread #15
	at DSL.node(node block appears to be neither running nor scheduled)
	at WorkflowScript.run(WorkflowScript:51)
Thread #44
	at DSL.sh(completed process (code -1) in /var/jenkins_home/workspace/<snip>@2@tmp/durable-dde3893d on JNLP4-connect connection from <snip> (pid: 5019); recurrence period: 0ms)
	at WorkflowScript.run(WorkflowScript:35)
	at DSL.withCredentials(Native Method)
	at WorkflowScript.run(WorkflowScript:34)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:28)
Thread #46
	at DSL.sh(completed process (code -1) in /var/jenkins_home/workspace/<snip>@5@tmp/durable-a4094138 on JNLP4-connect connection from <snip> (pid: 5031); recurrence period: 0ms)
	at WorkflowScript.run(WorkflowScript:35)
	at DSL.withCredentials(Native Method)
	at WorkflowScript.run(WorkflowScript:34)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:28)
Thread #48
	at DSL.sh(completed process (code -1) in /var/jenkins_home/workspace/<snip>@6@tmp/durable-08c0eb95 on JNLP4-connect connection from <snip> (pid: 5049); recurrence period: 0ms)
	at WorkflowScript.run(WorkflowScript:35)
	at DSL.withCredentials(Native Method)
	at WorkflowScript.run(WorkflowScript:34)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:28)
Thread #50
	at DSL.sh(completed process (code -1) in /var/jenkins_home/workspace/<snip>@7@tmp/durable-7ed0248f on JNLP4-connect connection from <snip> (pid: 5061); recurrence period: 0ms)
	at WorkflowScript.run(WorkflowScript:35)
	at DSL.withCredentials(Native Method)
	at WorkflowScript.run(WorkflowScript:34)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:28)
Thread #52
	at DSL.sh(completed process (code -1) in /var/jenkins_home/workspace/<snip>@tmp/durable-9649dd8e on JNLP4-connect connection from <snip> (pid: 5079); recurrence period: 0ms)
	at WorkflowScript.run(WorkflowScript:35)
	at DSL.withCredentials(Native Method)
	at WorkflowScript.run(WorkflowScript:34)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:28)
Thread #54
	at DSL.sh(completed process (code -1) in /var/jenkins_home/workspace/<snip>@tmp/durable-8e694ddb on JNLP4-connect connection from <snip> (pid: 5092); recurrence period: 0ms)
	at WorkflowScript.run(WorkflowScript:35)
	at DSL.withCredentials(Native Method)
	at WorkflowScript.run(WorkflowScript:34)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:28)
Thread #56
	at DSL.sh(completed process (code -1) in /var/jenkins_home/workspace/<snip>@10@tmp/durable-363241d9 on JNLP4-connect connection from <snip> (pid: 5107); recurrence period: 0ms)
	at WorkflowScript.run(WorkflowScript:35)
	at DSL.withCredentials(Native Method)
	at WorkflowScript.run(WorkflowScript:34)
	at DSL.node(running on builder1)
	at WorkflowScript.run(WorkflowScript:28)