Support Docker's credsStore (#591)

This way for #581 we don't need to specify a fully quialified URL and can simply use the following `~/.docker/config.json`:

```json
{
	"credsStore": "ecr-login"
}
```

Related to https://github.com/docker/cli/issues/2928

Sources/tart/Credentials/DockerConfigCredentialsProvider.swift

@@ -15,6 +15,10 @@ class DockerConfigCredentialsProvider: CredentialsProvider {
       return try executeHelper(binaryName: "docker-credential-\(helperProgram)", host: host)
     }
 
+    if let defaultCredsStore = config.credsStore {
+      return try executeHelper(binaryName: "docker-credential-\(defaultCredsStore)", host: host)
+    }
+
     return nil
   }
 
@@ -59,6 +63,7 @@ class DockerConfigCredentialsProvider: CredentialsProvider {
 struct DockerConfig: Codable {
   var auths: Dictionary<String, DockerAuthConfig>? = Dictionary()
   var credHelpers: Dictionary<String, String>? = Dictionary()
+  var credsStore: String? = nil
 }
 
 struct DockerAuthConfig: Codable {