[additionalimagestores] Simplify --storage-opt/--root syntax

[srcshelton]

Is this a BUG REPORT or FEATURE REQUEST? (leave only one on its own line)

/kind feature

Description

Currently, the --root option appears to have little functionality without also specifying --storage-opt= to clear the existing storage configuration (which includes the default root).

If use of --root implied --storage-opt= --root then podman's syntax would be clearer.

Note, however, that podman currently doesn't accept, e.g.

--storage-opt= --storage-opt="overlay.imagestore=<dir2>" --root <dir1>

(… as an error is generated), so either this would need to be relaxed or --root should only imply --storage-opt= if no additional --storage-opt value is specified.

A further enhancement would be to provide an option such as --swap-root <dir> or --root-swap <dir> which would be equivalent to:

podman --storage-opt="overlay.imagestore=${GRAPH_ROOT}" --root "${IMAGE_ROOT}"

(e.g. operate on a specified additiopalimagestore location but retain access to resources in the default graphRoot store)

Finally, since multiple additionalimagestore locations can be defined, the above syntax would be ideal to represent the case there one of the additional stores should be promoted to being the main store, whilst retaining access to both the original main store and all other defined additional stores.

See #9852 for further context.