Make MAILGUN_URL required field and update README (#12)

Since there's no handling for optionality of MAILGUN_URL

---------

Signed-off-by: Igor Shishkin <me@teran.dev>

README.md

@@ -27,3 +27,8 @@ docker run -it \
   -e RELAY_MAX_RECIPIENTS=50 \
   ghcr.io/teran/relay:latest
 ```
+
+RELAY_MAILGUN_URL is an API URL:
+
+* `https://api.eu.mailgun.net/v3` for EU
+* `https://api.mailgun.net/v3` for US

cmd/relay/main.go

@@ -28,7 +28,7 @@ type config struct {
 	AuthDisabled      bool      `envconfig:"AUTH_DISABLED"`
 	Domain            string    `envconfig:"DOMAIN" required:"true"`
 	MailgunAPIKey     string    `envconfig:"MAILGUN_API_KEY" required:"true"`
-	MailgunURL        string    `envconfig:"MAILGUN_URL"`
+	MailgunURL        string    `envconfig:"MAILGUN_URL" required:"true"`
 	MaxMessageBytes   int64     `default:"1048576" envconfig:"MAX_MESSAGE_BYTES"`
 	MaxRecipients     int       `default:"50" envconfig:"MAX_RECIPIENTS"`
 	MetricsAddr       string    `envconfig:"METRICS_ADDR" default:":8081" `
@@ -51,10 +51,10 @@ func main() {
 		"build_timestamp": buildTimestamp,
 	}).Infof("initializing application")
 
-	mg := mg.NewMailgun(cfg.Domain, cfg.MailgunAPIKey)
-	mg.SetAPIBase(cfg.MailgunURL)
+	mgCli := mg.NewMailgun(cfg.Domain, cfg.MailgunAPIKey)
+	mgCli.SetAPIBase(cfg.MailgunURL)
 
-	dr := mailgun.New(mg)
+	dr := mailgun.New(mgCli)
 	be := smtpWrapper.NewBackend(ctx, dr)
 
 	s := smtp.NewServer(be)