Clean up docker.pid file in /var/run/user

[killenheladagen]

Clean up docker.pid file in /var/run/user instead of in the old rooted location of /var/run/docker.pid

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• I have performed a self-review of my own code
• I have verified that the code builds perfectly fine on my local system
• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have verified that my code follows the style already available in the repository
• I have made corresponding changes to the documentation