Also looks for registry credential files in XDG folders

[sv3ndk]

This PR is my proposition for addressing #3524 . This is my first contribution to this project, don't hesitate to let me know if I failed to follow any convention.

This PR adds 2 locations, based on the XDG base directory specification, to the list of places where Jib looks for a credential file for registry authentication.

The main motivation for this change is enabling to use Jib together with Podman.

The 2 added locations are also mentioned in man containers-auth.json:

A credentials file in JSON format used to authenticate against container image registries. 
The primary (read/write) file is stored at ${XDG_RUNTIME_DIR}/containers/auth.json on Linux; 
on Windows and macOS, at $HOME/.config/containers/auth.json.

When searching for the credential for a registry, the following files will be read in sequence 
until the valid credential is found: first reading the primary (read/write) file, or the explicit override 
using an option of the calling application.  If credentials are not present, search in 
${XDG_CONFIG_HOME}/containers/auth.json, $HOME/.docker/config.json, $HOME/.dockercfg.
...

This logic seems aligned with the philosophy of Jib, which already iterates through various locations.

Jib also already follows XDG specification, for example for storing its own config.

The resulting flow should hopefully be natural to most users: both Docker and Podman users would have their credential file picked up automatically in the default location of their tool.

I updated documentation to mention the new configuration feature.

[ddixit14]

@sv3ndk Thank you for your contribution, really appreciate it. The team will review this after the holidays.
Happy Coding 👯

[sv3ndk]

Thanks for the kind feed-back. Enjoy the holidays :)

[ddixit14]

You too. Happy Holidays!

[sv3ndk]

Hi @ddixit14 , I pushed a commit to fix the formatting issues, could you please re-trigger the CI workflow?

[sv3ndk]

Sorry, I'm realizing there was one remaining formatting issue that ./gradlew clean goJF had not fixed (I probably forgot the to "clean" first).
Now hopefully it should be correct. At least ./gradlew clean jib-plugins-common:verifyGoogleJavaFormat does not report anything on my host atm.

[chanseokoh]

Thanks for your contribution. I'm going to merge this once the tests pass, unless I hear back any concerns from you.

[sonarqubecloud]

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