https://github.com/docker/docker/blob/97bed96759678c3611a6cacb533783b0f55fa67b/utils/names.go#L6 defines valid Docker volume names as matching the regexp ^[a-zA-Z0-9][a-zA-Z0-9_.-]+$. The definition in main.ValidVolumeName omits underscores and periods in the 2nd character and onwards.
https://github.com/docker/docker/blob/97bed96759678c3611a6cacb533783b0f55fa67b/utils/names.go#L6 defines valid Docker volume names as matching the regexp
^[a-zA-Z0-9][a-zA-Z0-9_.-]+$. The definition inmain.ValidVolumeNameomits underscores and periods in the 2nd character and onwards.