Fix blocking image observer

[chiting]

I'm not very good with python but I figured this should fix some issues happening when running locally with Podman.

I'm also not sure how it behaves with Docker or other alternatives (like Colima).

Which issue(s) does this change fix?

#7298

Why is this change necessary?

Calling the events function from the main thread will block until it receives data as docker-py uses urllib3.openurl under the hood which is blocking (read).

It can be replicated using curl:

# my docker.sock is a symlink to podman API socket
% curl -vv --unix-socket /var/run/docker.sock 'http://localhost/v1.35/events?filters=%7B%22type%22%3A+%5B%22image%22%5D%7D'

The above will block until it receives data.

How does it address the issue?

We don't want to block the main thread, so moving the self.events assignment in a thread (the function that actually watches for changes) seems to be the solution.

What side effects does this change have?

I can't see any side effect this would introduce.

Mandatory Checklist

PRs will only be reviewed after checklist is complete

• Add input/output type hints to new functions/methods
• Write design document if needed (Do I need to write a design document?)
• Write/update unit tests
• Write/update integration tests
• Write/update functional tests if needed
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.