Preserve volumeMounts from PodConfig

[ahyattdev]

Summary

Preserve volumeMounts from PodConfig.

This currently handled for Check and Prune, but not Backup, Archive, or Restore.

Issue mentioned here in a comment: #584 (comment)

One use case: if securityContext calls for a read only root FS, you need an emptyDir at /.cache for the restic cache to be written to.

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[ahyattdev]

Just saw #1056, which adds an emptyDir for /.cache by default now. That was my use case. So closing this PR.

[beniwtv]

I would still want this, for other use-cases. We use a CSI volume for secrets (for example, for S3), so being able to specify additional volumes to be included would still be beneficial.