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.

Which issue(s) does this change fix?

aws#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).

How does it address the issue?

We don't want to block the main thread, so moving the self.events assignment in a thread 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.