Schedule docker containers for removal.

Instead of removing docker containers right after reap, schedule it to
be removed later.

Review: https://reviews.apache.org/r/26861
Fixed docker flaky tests.

Docker tests are flaky, mostly around getting expected output from the
docker container forwarded to stdout/stderr.

This is due to Docker not always have the stdout/stderr output
available for docker logs if kill/rm is called.

Review: https://reviews.apache.org/r/26862