Add hint if provider is called outside of CGI

Co-authored-by: Sascha L. Teichmann <sascha.teichmann@intevation.de>

cmd/csaf_provider/main.go

@@ -13,6 +13,7 @@ import (
 	"log"
 	"net/http"
 	"net/http/cgi"
+	"os"
 
 	"github.com/csaf-poc/csaf_distribution/util"
 	"github.com/jessevdk/go-flags"
@@ -22,6 +23,16 @@ type options struct {
 	Version bool `long:"version" description:"Display version of the binary"`
 }
 
+const cgiRequired = "The csaf_provider is a cgi binary and is designed to be served via a web server."
+
+func ensureCGI() {
+	if _, ok := os.LookupEnv("REQUEST_METHOD"); !ok {
+		fmt.Println(cgiRequired)
+		fmt.Println("Version: " + util.SemVersion)
+		os.Exit(1)
+	}
+}
+
 func main() {
 	var opts options
 	parser := flags.NewParser(&opts, flags.Default)
@@ -31,6 +42,8 @@ func main() {
 		return
 	}
 
+	ensureCGI()
+
 	cfg, err := loadConfig()
 	if err != nil {
 		cgi.Serve(http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {