Make dockerd 'allow start' flag thread safe

[github-actions]

This is an automatic cherry-pick of c51cb051e8422d507f2f5e8ed7213a618e2ac52b

The original PR was #202 with message:

HTTP requests are handled by another thread.

A successful file upload must allow dockerd to recover.

Checklist before requesting a review

• 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