[gatus] fix config volume

[angelnu]

Description of the change

Harcoded config volume in common.yaml had several issues. As result the passed config was ignored

Benefits

Fix gatus chart

Possible drawbacks

None

Applicable issues

• fixes NA

Additional information

Checklist

• [x] Title of the PR starts with chart name (e.g. [home-assistant])
• [x] Chart version bumped in Chart.yaml according to semver.
• [x] Chart artifacthub.io/changes changelog annotation has been updated in Chart.yaml. See Artifact Hub documentation for more info.
• [x] Variables have been documented in the values.yaml file.