Use PORT environment variable (#6)

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.11.1-alpine AS builder
+FROM golang:1.12-alpine AS builder
 RUN apk update && apk add --no-cache git gcc musl-dev
 WORKDIR /build
 COPY . .

README.md

@@ -64,6 +64,13 @@ Download the latest release and run the command:
 ./jira-to-slack
 ```
 
+It binds port 3000 by default.
+You can set the port by `PORT` environment variable.
+
+```sh
+PORT=8080 ./jira-to-slack
+```
+
 ### Docker
 
 Run the Docker image as follows:

main.go

@@ -12,6 +12,8 @@ import (
 
 var version string
 
+const defaultPort = "3000"
+
 func router() http.Handler {
 	r := mux.NewRouter()
 	r.Handle("/", &handlers.Index{}).Methods("GET")
@@ -25,7 +27,13 @@ func router() http.Handler {
 
 func main() {
 	log.Printf("jira-to-slack %s", version)
-	addr := ":3000"
+
+	port := os.Getenv("PORT")
+	if port == "" {
+		port = defaultPort
+	}
+	addr := ":" + port
+
 	log.Printf("Listening on %s", addr)
 	if err := http.ListenAndServe(addr, router()); err != nil {
 		log.Fatalf("Error while listening on %s: %s", addr, err)