Today I learned you can label Docker containers with any key=value pair when creating them:

docker run --label com.vintageaerial.group="worker" -d vintageaerial/vintageaerial:latest bundle exec rake jobs:work

Then you can filter the output of docker ps to only include said labels:

docker ps --filter "label=com.vintageaerial.group=worker"

The documentation seems to favor using reverse DNS-style labels as a namespace.