Prevent gc from closing image file descriptor

[DrDaveD]

Description of the Pull Request (PR):

This prevents garbage collection from closing a container image file descriptor, by initializing the Image.File object when it is first referenced rather than just before use. Previously if a copy of the image file descriptor was made (which is easy to do accidentally in go), the File object would be created in that copy and so when the copy was dereferenced garbage collection would close the corresponding file descriptor.

This fixes or addresses the following GitHub issues:

• Fixes Using -u --nv prevents singularity from holding a file descriptor on the container sandbox #6047
• Replaces Fix image file reference to prevent premature close by Go garbage collector #6052

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