internal/cli/cmd: report latest stable image version

As of cilium/cilium#16453 cilium provides a
stable.txt file in the master branch pointing to the latest stable
version. Use it to report that latest stable version in `cilium
version`.

Suggested-by: Robin Hahling <robin.hahling@gw-computing.net>
Signed-off-by: Tobias Klauser <tobias@cilium.io>

internal/cli/cmd/version.go

@@ -16,7 +16,10 @@ package cmd
 
 import (
 	"fmt"
+	"io"
+	"net/http"
 	"runtime"
+	"strings"
 
 	"github.com/cilium/cilium-cli/defaults"
 
@@ -33,6 +36,21 @@ var (
 	GitHash string
 )
 
+func getLatestStableVersion() string {
+	resp, err := http.Get("https://raw.githubusercontent.com/cilium/cilium/master/stable.txt")
+	if err != nil {
+		return "unknown"
+	}
+	defer resp.Body.Close()
+
+	b, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "unknown"
+	}
+
+	return strings.TrimSpace(string(b))
+}
+
 func newCmdVersion() *cobra.Command {
 	return &cobra.Command{
 		Use:   "version",
@@ -50,6 +68,7 @@ func newCmdVersion() *cobra.Command {
 			// the cluster, if any. See https://github.com/cilium/cilium-cli/issues/131
 			fmt.Printf("cilium-cli: v%s%s compiled with %v on %v/%v\n", Version, gitInfo, runtime.Version(), runtime.GOOS, runtime.GOARCH)
 			fmt.Printf("cilium image (default): %s\n", defaults.Version)
+			fmt.Printf("cilium image (stable): %s\n", getLatestStableVersion())
 		},
 	}
 }