Add more documentation/examples for --regex parameter

[oliver]

What would you like to be added: It would be nice to have more documentation and examples for the --regex command line parameter.

Why is this needed: I did not get this parameter to work; and from looking at the code I could not find out where the regex check is implemented. The feature was originally added in https://github.com/030/n3dr/commit/c3325c53636e3e6f4014dbd3f7a6696bb416091f , but I did not manage to find similar code in the latest main branch.

Example that might be good to add to the docs: how could the --regex parameter be used to export only certain Docker images (e.g. those that look like ubuntu:22)? And do I have to use ^ and $ at the beginning and end of the regex?