clean-docker: Add clean dangling containers instruction

To stop integration tests running containers.

The added instruction has been executed on the node with success [1]

[1]

root@jenkins-docker02:~# docker ps --format='table {{.ID}}\t{{.Names}}\t{{.Status}}' | grep "swh_test" | grep "hours\|days\|months" | awk '{print $1}' | xargs docker container stop
71cbbbb07415
428c854f9f98
...
6fc557e614d6
1b3ac02777ce
8d80b89e650d

Refs. swh/infra/sysadm-environment#5198 (closed)