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

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.

[EndpointFromContext]: 33494921b8/cli/context/docker/load.go (L139-L149)
[docker.EndpointMeta]: 33494921b8/cli/context/docker/load.go (L19-L21)
[newAPIClientFromEndpoint]: 33494921b8/cli/command/cli.go (L295-L305)
[TestInitializeFromClient]: 33494921b8/cli/command/cli_test.go (L157-L205)

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/cli.go

@@ -59,7 +59,9 @@ type Cli interface {
 }
 
 // DockerCli is an instance the docker command line client.
-// Instances of the client can be returned from NewDockerCli.
+// Instances of the client should be created using the [NewDockerCli]
+// constructor to make sure they are properly initialized with defaults
+// set.
 type DockerCli struct {
 	configFile         *configfile.ConfigFile
 	options            *cliflags.ClientOptions
@@ -74,7 +76,7 @@ type DockerCli struct {
 	init               sync.Once
 	initErr            error
 	dockerEndpoint     docker.Endpoint
-	contextStoreConfig store.Config
+	contextStoreConfig *store.Config
 	initTimeout        time.Duration
 	res                telemetryResource
 
@@ -250,13 +252,33 @@ func (cli *DockerCli) Initialize(opts *cliflags.ClientOptions, ops ...CLIOption)
 		return errors.New("conflicting options: cannot specify both --host and --context")
 	}
 
+	if cli.contextStoreConfig == nil {
+		// This path can be hit when calling Initialize on a DockerCli that's
+		// not constructed through [NewDockerCli]. 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).
+		//
+		// [EndpointFromContext]: https://github.com/docker/cli/blob/33494921b80fd0b5a06acc3a34fa288de4bb2e6b/cli/context/docker/load.go#L139-L149
+		if err := WithDefaultContextStoreConfig()(cli); err != nil {
+			return err
+		}
+	}
+
 	cli.options = opts
 	cli.configFile = config.LoadDefaultConfigFile(cli.err)
 	cli.currentContext = resolveContextName(cli.options, cli.configFile)
 	cli.contextStore = &ContextStoreWithDefault{
-		Store: store.New(config.ContextStoreDir(), cli.contextStoreConfig),
+		Store: store.New(config.ContextStoreDir(), *cli.contextStoreConfig),
 		Resolver: func() (*DefaultContext, error) {
-			return ResolveDefaultContext(cli.options, cli.contextStoreConfig)
+			return ResolveDefaultContext(cli.options, *cli.contextStoreConfig)
 		},
 	}