Add PVC and volumeMounts for deployment

[phoenix-bjoern]

This is a follow up on issue #2. The PR allows to serve files from localy attached volume (e.g. NFS shares or RWX).
The PR provides configurations for persistence. When enabled a PVC will be created. Also an existing PVC can be used instead. The PVC will get defined as a volume in the imgproxy deployment and the PV will be mounted at the specified mountPath.

It makes sense to configure IMGPROXY_LOCAL_FILESYSTEM_ROOT separately in the values.yaml:

features:
    security:
      allowedSources: "local://"
    custom:
      IMGPROXY_LOCAL_FILESYSTEM_ROOT: "/images"

[nepalez]

@phoenix-bjoern Thank you! Will look at it on the weekend