mailers sidekiq queue has some sort of reliance on shared storage

Problem Statement

During an evaluation of various Sidekiq Queues that are slated to be migrated into Kubernetes, it was discovered the queue mailers has some form of reliance on shared storage. In the case of GitLab.com this involves reading/writing data to an NFS share. gitlab-com/gl-infra/delivery#1046 (comment 409416599)

Due to this, the queue was removed from our testing and will not yet be migrated into Kubernetes.

It is not known whether or not the mailers queue is using NFS for reading shared data, or if it is simply a scratch space for temporary files while it is performing work.

Closing Criterion

• Determine ownership of this queue
  • https://gitlab.com/gitlab-org/gitlab/-/blob/ce5b73651ed3c83931fb7f1215cf3324db74a3a7/app/workers/all_queues.yml#L1471
• Determine where/why this queue uses shared storage
• Come up with a plan of action to remove the dependency on shared storage