Verify the set of machines that have docker installed and update labels

We currently have 5 machines labelled with docker for use by test: https://ci.adoptopenjdk.net/label/ci.role.test&&hw.arch.x86&&sw.os.linux&&sw.tool.docker/

Given that docker is in the ansible playbooks, I think there are actually more machines with docker installed than are labelled with sw.tool.docker. We should go through the set of machines and try to run the external (docker-based tests from the openjdk-tests repo) to verify which additional test machines can be used for external tests.

Start with the CI label query in the issue and the Ansible playbooks that install Docker. Check each candidate machine by running the external Docker-based tests from the openjdk-tests repository. Done means the machines that pass are verified and their sw.tool.docker labels are updated.

Written by the indexing model from the issue text.