Allow absolute path in credential store/helpers

[hamnis]

Implements #6103

Allows users to use a credential helper that does not live in path. The underlying credentials helper library uses os/exec exec.Command so this should be safe to do.

I should add a test, please advice for the best location to do that.

- What I did
Checks if the path is absolute, then return that instead of prepending docker-credential-

- How I did it
uses filepath to check if the path is absolute

- How to verify it
use an absolute path to a binary file which implements the docker-credential protocol.

- Human readable description for the release notes

Allows users to use a credential helper that does not live in path.

[laurazard]

Just a nit:

[hamnis]

Thanks @laurazard. I am not really a go programmer

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 55.02%. Comparing base (b2c4452) to head (e5ff2f8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6104      +/-   ##
==========================================
- Coverage   55.04%   55.02%   -0.03%     
==========================================
  Files         361      361              
  Lines       30142    30145       +3     
==========================================
- Hits        16591    16586       -5     
- Misses      12594    12601       +7     
- Partials      957      958       +1     

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[thaJeztah]

In general, I wonder if you could provide more context on why you need this change; ultimately, the CLI would look for binaries in PATH, so it would already be possible to update your PATH if you want custom locations to take precedence (which .. is basically what PATH was designed for).

That's not a "hard reject" for this change, but we're also working on various changes around authentication (and credential stores); I want to avoid introducing more complexity if it can get in the way of those changes (especially if it's for a niche-case that already could have workarounds (like PATH mentioned above)).

[hamnis]

Currently i am using docker in a setting where I need to generate the configuration file and set credentials to docker repos as part of this generation.

Since I have to then also generate a shell script to piggyback to my own tooling and manipulate the path is annoying, but not a showstopper. Since I cant enforce how my users set their PATH variables, I have to build a shim on top of docker to set the PATH correctly.

This would help by making this interaction a bit less annoying.

I also think users would like to be able to specify absolute paths to these authentication tools.