Update manifest / config for correct overlay digest after changes

[dtrudg]

The image will be correct at the point the overlay is initially added. However, as the overlay is modified the manifest / config will no longer be correct as the true content hash of the overlay will change as it is modified.

We must have a way to reconcile the manifest / config with updated overlay content - this effectively makes a new OCI image within the OCI-SIF.

We must perform the reconciliation, at minimum, when an image is pushed to a registry for distribution. It could be possible to update the manifest/config whenever a container execution that modifies the r/w overlay exits... but this is potentially problematic where jobs may be killed, OCI-SIF files are stored on cluster-wide filesystems etc.

[dtrudg]

Going to implement this as an explicit singularity overlay sync command first. Will then later consider automating this on push.