doc: allow commit prefix 'docker:' for container generation code

[sxa]

Noted during #4393 that the docker: prefix wasn't listed as valid in the contributing documentation.

This should be used for anything related to container generation e.g. things in ansible/docker or potentially the dockerstatic images since those are used be the jenkins files in ansible/docker e.g. by the test_image_updater job.

Not that this is already in widespread use and was first used in a commit from 2020.

Checklist

• commit message has one of the standard prefixes
• faq.md updated if appropriate
• other documentation is changed or added (if applicable)
• playbook changes run through VPC or QPC (if you have access)
• VPC/QPC not applicable for this PR
• for inventory.yml changes, bastillion/nagios/jenkins updated accordingly