Fix dockerfile to use env

Dockerfile

@@ -5,6 +5,6 @@ WORKDIR ${GOPATH}/src/git.jonasfranz.software/JonasFranzDEV/mensabot
 RUN go build
 
 FROM alpine:3.7
-
+ENV MENSABOT_TOKEN
 COPY --from=build-env /go/src/git.jonasfranz.software/JonasFranzDEV/mensabot/mensabot /app/mensabot/mensabot
 ENTRYPOINT ["/app/mensabot/mensabot"] 

bot.go

@@ -31,7 +31,7 @@ var (
 func main() {
 	token := os.Getenv("MENSABOT_TOKEN")
 	if len(token) == 0 {
-		println("Please add token as cli argument.")
+		println("Please add token as environment variable: MENSABOT_TOKEN.")
 		return
 	}
 	dg, err := discordgo.New(fmt.Sprintf("Bot %s", token))