Refactor Docker Auth to improve testability and reliability

Description

Followup of !2048 (merged).

The MR only moved auth logic from the docker executor to a separate package and avoided refactoring it to limit the risk of regressions.

The code however suffers from some reliability and testability issues - using global variables, not handling errors, low test coverage.

Proposal

Refactor the code in helpers/docker/auth to:

• wrap the functions in a struct, possibly with an interface so that it can be injected in executors and mocked
• inject HomeDirectory as a dependency to the struct to avoid setting it as a global variable in tests
• handle errors from readConfigsFromReader
• Convert TestSplitDockerImageName, TestSplitDefaultDockerImageName and TestSplitDefaultIndexDockerImageName to table driven tests and add more cases.
• Inject userFetcher so that tests can be created for the logic fetching the user and using it to determine the home directory.
• increase test coverage for resolver functions - readConfigsFromReader, readConfigsFromCredentialsStore, readConfigsFromCredentialsHelper