Fallback to default context when config file references a missing context

Previously, when the "CurrentContext" field of the config file referenced a context that did not exist in the store,
the CLI would fail (even for simple things like docker context ls).
This causes an issue with Docker Desktop "Contexts Syncing" feature (that uses the same store between win32 and WSL2):
If win32 cli removes the "current context" of the WSL 2 CLI (which it can do if it is not the "current context" in
the win32 config file), it would make the WSL 2 CLI unusable, asking the user to modify the config file by hand.

This commit relaxes this behavior a bit: explicitly referencing a missing context (using the --context flag or
DOCKER_CONTEXT env var) will still produce an error, but implicitly referencing such a missing context (via the
CurrentContext config field) will fallback to use the "default" context.

Signed-off-by: Simon Ferquel <simon.ferquel@docker.com>

Author: simonferquel

cli/command/cli.go

@@ -549,7 +549,9 @@ func resolveContextName(opts *cliflags.CommonOptions, config *configfile.ConfigF
 	if config != nil && config.CurrentContext != "" {
 		_, err := contextstore.GetMetadata(config.CurrentContext)
 		if store.IsErrContextDoesNotExist(err) {
-			return "", errors.Errorf("Current context %q is not found on the file system, please check your config file at %s", config.CurrentContext, config.Filename)
+			// If config file references a context that does not exist, fallback to default
+			// Explicitly requested contexts (flag or env-var based) will still get the error
+			return DefaultContextName, nil
 		}
 		return config.CurrentContext, err
 	}