Docker: Add insecure registry support

For security reasons, docker-py will only fetch images from HTTPS, not plain HTTP
based registries.

This change adds a insecure_registry flag to the docker module to allow
it to fetch images also via insecure HTTP.

For (my) convenience, this branch also includes my other PRs:
- #8815 
- #8805 

Let me know if you want branches with only those commits - or just cherry-pick the commits.