github-runner docs: Add custom labels

How to add custom labels

This charm supports adding custom labels to the runners.

By using juju config to change the charm configuration labels, additional custom labels can be attached to the self-hosted runners.

juju config <APP_NAME> labels=<COMMA_SEPARATED_LABELS>

Examples of a COMMA_SEPARATED_LABELS value include “large,gpu” and “small,arm64”. Accepted values are alphanumeric values with underscores (_); whitespaces before and after the the word will be automatically trimmed.