Add support for KUBECONFIG env variable (#44)

Author: sliemeobn

README.md

@@ -120,6 +120,7 @@ client.shutdown(queue: queue) { (error: Error?) in
 
 The client tries to resolve a `kube config` automatically from different sources in the following order:
 
+- If set, kube config file at path of environment variable `KUBECONFIG`
 - Kube config file in the user's `$HOME/.kube/config` directory 
 - `ServiceAccount` token located at `/var/run/secrets/kubernetes.io/serviceaccount/token` and a mounted CA certificate, 
 - if it's running in Kubernetes.

Sources/SwiftkubeClient/Client/KubernetesClient.swift

@@ -108,6 +108,7 @@ public actor KubernetesClient {
 	///
 	/// The client tries to resolve a `kube config` automatically from different sources in the following order:
 	///
+	/// - A Kube config file at path of environment variable `KUBECONFIG` (if set)
 	/// - A Kube config file in the user's `$HOME/.kube/config` directory
 	/// - `ServiceAccount` token located at `/var/run/secrets/kubernetes.io/serviceaccount/token` and a mounted CA certificate, if it's running in Kubernetes.
 	///

Sources/SwiftkubeClient/Config/KubernetesClientConfig.swift

@@ -286,12 +286,19 @@ internal struct LocalKubeConfigLoader: KubernetesClientConfigLoader {
 		redirectConfiguration: HTTPClient.Configuration.RedirectConfiguration,
 		logger: Logger?
 	) throws -> KubernetesClientConfig? {
-		guard let homePath = ProcessInfo.processInfo.environment["HOME"] else {
-			logger?.info("Skipping kubeconfig in $HOME/.kube/config because HOME env variable is not set.")
+		var kubeConfigURL: URL?
+
+		if let kubeConfigPath = ProcessInfo.processInfo.environment["KUBECONFIG"] {
+			kubeConfigURL = URL(fileURLWithPath: kubeConfigPath)
+		} else if let homePath = ProcessInfo.processInfo.environment["HOME"] {
+			kubeConfigURL = URL(fileURLWithPath: homePath + "/.kube/config")
+		}
+
+		guard let kubeConfigURL else {
+			logger?.info("Skipping local kubeconfig detection, neither environment variable KUBECONFIG nor HOME are set.")
 			return nil
 		}
 
-		let kubeConfigURL = URL(fileURLWithPath: homePath + "/.kube/config")
 		return try? URLConfigLoader(url: kubeConfigURL)
 			.load(timeout: timeout, redirectConfiguration: redirectConfiguration, logger: logger)
 	}