cli/command: DockerCli.Initialize: make sure context-store config is set

[thaJeztah]

In most situations, the CLI is created through the NewDockerCli constructor, however, it's possible to construct a CLI manually (&DockerCli{}). We should probably prevent this (and un-export the DockerCli implementation), but currently have some code-paths that depend on the type being exported.

When constructing the CLI with this approach, the CLI would not be fully initialized and not have the context-store configuration set up.

Using the default context store without a config set will result in Endpoints
from contexts not being type-mapped correctly, and used as a generic
map[string]any, instead of a docker.EndpointMeta.

When looking up the API endpoint (using EndpointFromContext), no endpoint will be found, and a default, empty endpoint will be used instead which in its turn, causes newAPIClientFromEndpoint to be initialized with the default config instead of settings for the current context (which may mean; connecting with the wrong endpoint and/or TLS Config to be missing).

I'm not sure if this situation could happen in practice, but it caused some of our unit-tests (TestInitializeFromClient among others) to fail when running outside of the dev-container on a host that used Docker Desktop's "desktop-linux" context. In that situation, the test would produce the wrong "Ping" results (using defaults, instead of the results produced in the test).

This patch:

• updates the contextStoreConfig field to be a pointer, so that we are able to detect if a config was already set.
• updates the Initialize function to set the default context-store config if no config was found (technically the field is mostly immutable, and can only set through WithDefaultContextStoreConfig, so this may be slightly redundant).

We should update this code to be less error-prone to use; the combination of an exported type (DockerCli), a constructor NewDockerCli and a Initialize function (as well as some internal contructors to allow lazy initialization) make constructing the "CLI" hard to use, and there's various codepaths where it can be in a partially initialized state. The same applies to the default context store, which also requires too much "domain" knowledge to use properly.

I'm leaving improvements around that for a follow-up.

- Human readable description for the release notes

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.81818% with 4 lines in your changes missing coverage. Please review.

Project coverage is 59.13%. Comparing base (3349492) to head (ed05112).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5983      +/-   ##
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+ Coverage   59.12%   59.13%   +0.01%     
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  Files         355      355              
  Lines       29740    29759      +19     
==========================================
+ Hits        17583    17599      +16     
- Misses      11182    11184       +2     
- Partials      975      976       +1     

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