cmd: rename -env to -envfile and use -env to print the environment (#2517)

* caddy: Rename env flag to envfile

* caddy: Add env flag to print environment variables

caddy/caddymain/run.go

@@ -53,7 +53,8 @@ func init() {
 	flag.StringVar(&disabledMetrics, "disabled-metrics", "", "Comma-separated list of telemetry metrics to disable")
 	flag.StringVar(&conf, "conf", "", "Caddyfile to load (default \""+caddy.DefaultConfigFile+"\")")
 	flag.StringVar(&cpu, "cpu", "100%", "CPU cap")
-	flag.StringVar(&envFile, "env", "", "Path to file with environment variables to load in KEY=VALUE format")
+	flag.BoolVar(&printEnv, "env", false, "Enable to print environment variables")
+	flag.StringVar(&envFile, "envfile", "", "Path to file with environment variables to load in KEY=VALUE format")
 	flag.BoolVar(&fromJSON, "json-to-caddyfile", false, "From JSON stdin to Caddyfile stdout")
 	flag.BoolVar(&plugins, "plugins", false, "List installed plugins")
 	flag.StringVar(&certmagic.Email, "email", "", "Default ACME CA account email address")
@@ -117,6 +118,12 @@ func Run() {
 		mustLogFatalf("%v", err)
 	}
 
+	if printEnv {
+		for _, v := range os.Environ() {
+			fmt.Println(v)
+		}
+	}
+
 	// initialize telemetry client
 	if EnableTelemetry {
 		err := initTelemetry()
@@ -580,6 +587,7 @@ var (
 	toJSON          bool
 	version         bool
 	plugins         bool
+	printEnv        bool
 	validate        bool
 	disabledMetrics string
 )