Bug: Build Skipping prevents Docker cleanup

My buildkite jobs have lately been failing due to an inability to provision enough Docker network interfaces, similar to https://github.com/buildkite/agent/issues/277. However, most of our jobs successfully clean up their network interfaces.

I discovered that all of the jobs that failed to clean up their network interfaces failed to do so because the corresponding container was still running, and all of those containers corresponded to jobs that were canceled after a few minutes due to build skipping settings.

I haven't verified whether or not there's a zombie container every time build skipping happens.

Let me know if you need any additional information!

Start by reproducing a Buildkite job canceled through build skipping while using Docker, then compare its cleanup behavior with jobs that complete normally. Confirm the issue is fixed when canceled jobs no longer leave their containers or Docker network interfaces running.

Written by the indexing model from the issue text.