Communication with DockerHub is changed to use it's API instead of the Docker Engine.

[janosmurai]

Type Of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Any modification in the test cases
• Any modification in the documentation

Checklist:

• I have read and followed the contribution guideline.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• All the test cases pass and new modifications in the production code are 100% covered.
• I have made corresponding changes to the documentation.

Related Issue

Closing:
DEM-284
#203

Description

When obtaining the available repositories in a DockerHub namespace, the Container
Engine's search functionality had been used. This somehow had changed, so
filtering for the namespace didn't work anymore.
The new solution is to use the DockerHub's API directly.

To be able to manage the registries properly the config file got extended so
the registry config has a new field called 'namespace'. This breaks backward
compatibility.

How Has This Been Tested?

Tested on Ubuntu 22.04