Support plaintext credentials as multi-call binary

[laurazard]

- What I did

The problem

The Docker CLI supports storing/managing credentials without a credential-helper, in which case credentials are fetched from/saved to the CLI config file (~/.docker/config.json). This is all managed entirely by the CLI itself, without resort to a separate binary.

There are a few issues with this approach – for one, saving the credentials together with all the configurations make it impossible to share one without the other, so one can't for example bind mount the config file into a container without also including all configured credentials.

Another issue is that this has made it so that any other clients accessing registry credentials (such as https://github.com/google/go-containerregistry) all have to both:

• read/parse the CLI config.json, to check for credentials there, which also means they're dependent on this type and might break if the type changes/we need to be careful not to break other codebases parsing this file, and can't change the location where plaintext credentials are stored.
• support the credential helper protocol, so that they can access credentials when users do have configured credential helpers.

This means that if we want to do something like support oauth credentials by having credential-helpers refresh oauth tokens before returning them, we have to both implement that in each credential-helper and in the CLI itself, and any client directly reading config.json will also need to implement this logic.

The proposed solution

This commit turns the Docker CLI binary into a multicall binary, acting as a standalone credentials helper when invoked as docker-credential-file, while still storing/fetching credentials from the configuration file (~/.docker/config.json), and without any further changes.

This represents a first step into aligning the "no credhelper"/plaintext flow with the "credhelper" flow, meaning that instead of this being an exception where credentials must be read directly from the config file, credentials can now be accessed in the exact same way as with other credential helpers – by invoking docker-credential-[credhelper name], such as docker-credential-pass, docker-credential-osxkeychain or docker-credential-wincred.

This would also make it possible for any other clients accessing credentials to untangle themselves from things like the location of the credentials, parsing credentials from config.json, etc. and instead simply support the credential-helper protocol, and call the docker-credential-file binary as they do others.

Asides from addressing the aforementioned issues, the multi-call binary solution allows us to to do this while making minimal changes to packaging/delivery pipelines, as the CLI binary already has all the packaging setup required and we don't need to package or ship a separate binary. The only change required to use this is for existing packages to create a new symlink during installation, and this flow can be used. This also reduces overhead with managing a new repo, dependencies, etc.

- How I did it

Make the CLI binary multi-call, and act as a file credentials-helper when invoked as such.

- How to verify it

• ln -s [docker CLI binary] [somewhere in your path/docker-credential-file]
• docker-credential-file list

- Description for the changelog

todo

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 6.45161% with 29 lines in your changes missing coverage. Please review.

Project coverage is 60.85%. Comparing base (31eeed7) to head (d6ce046).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5536      +/-   ##
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+ Coverage   60.53%   60.85%   +0.32%     
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  Files         342      346       +4     
  Lines       23483    23522      +39     
==========================================
+ Hits        14216    14315      +99     
+ Misses       8292     8234      -58     
+ Partials      975      973       -2