use of persistence claim must add custom variable

when deploy imgproxy with and persistance mount the local_filesystem_root must be set manual.

```yaml
imgproxy:
  features:
    urlSignature:
      enabled: true
      key: xxxx
      salt: xxxx
    prometheus:
      enabled: true
    custom:
      IMGPROXY_LOCAL_FILESYSTEM_ROOT: /images  #<-- need to add this
  persistence:
    enabled: true
    existingClaim: private
```