parallel jobs not starting with docker workflow

[Marco Sacchetto]

Hi,

I've been trying (with no result for now) to define a parallel declarative pipeline such as this:
```

pipeline{

    agent any

    stages {

        stage('Configure projects') {

            parallel {

                stage('Configure 1') {

                    agent {

                        docker {

                            image '<myimage>'

                            alwaysPull true

                        }
                        steps { ..... }

                    }
                  stage('Configure 2') {
                  ........
                  }
               }
          }
.......
```
I'm uncurring in an issue that is very similar to https://issues.jenkins-ci.org/browse/JENKINS-46831 , with the big question mark due that 
the bug there is from 2017 and our plugins are updated.
The build arrives up to the parallel closure, and it spins up the parallel
jobs in other available agents. At that point it gets stuck. On blueocean
it seems stuck at the docker pull phase, but it doesn't look like it is
pulling anything. No logs appear in the logs window.
After around 5 minutes the build fails, and both stages print out

process apparently never started in /data1/jenkins_workingfolder/workspace/*******/*******@tmp/durable-9585b504 (running Jenkins temporarily with -Dorg.jenkinsci.plugins.durabletask.BourneShellScript.LAUNCH_DIAGNOSTICS=true might make the problem clearer)

Unluckily this is a managed service an I'm unable to run jenkins with
such an option activated. Anybody got an idea of what might be
happening here?

[Marco Sacchetto]

I'm replying to myself, in case this helps anybody else. This seems to be triggered by having a (valid) global variable defined for agents in Jenkin's main configuration. Once removed, things started working as expected.