in_podman_metrics: Added image label

[pabloxxl]

Added "image" label to all container metrics - it points to container image used by referenced container. Also, added new unit test which checks if out-of-order fields in json configuration file still work.

This is done mostly to be more consistent with cadvisor container metrics. In case of image missing in podman json config (I can't really see why it could happen), this label will point to "unknown"

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Testing
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  valgrind.txt - unit tests with valgrind on

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  Documentation PR

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