`docker_client.{containers,images}` data → conu metadata

[TomasTomecek]

#221 follow up

We now parse inspect data and transform it into conu metadata. We need to add one more transformation step and that would be docker_client.{containers,images}, those are different. These would be utilized when doing docker_backend.list_{images,container}.

• [ ] ideally utilize existing code which performs transformation
• [ ] write a test case a verify that all metadata from docker_client.{containers,images} is being processed correctly
• [ ] write a test case that metadata is being updated correctly once listed and updated with .get_metadata