Update DOCKER_IMAGE tag in testslurm.yml

[adi611]

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)

Summary

Update DOCKER_IMAGE tag in testslurm.yml from latest to 21.08.6

Checklist

• [ ] I have added tests to cover my changes (if necessary)
• [ ] I have updated documentation (if necessary)

[djarecka]

Thank you, perhaps you can add this to .github directory for the references