Difficulty: beginner
Estimated Time: 20-30 minutes

Welcome! In this lesson you will:

  • Build and run a Go application as a container
  • Write a Dockerfile to define how a container image gets built
  • Run a container, instantiating what's in the container image
  • See how a network port from the host's perspective can be mapped to a different port inside the container

Congratulations, you've finished this scenario! Here's what you have covered:

  • Building a container image from a Dockerfile
  • Running a container
  • Network port mapping

Container basics: containers and images

Step 1 of 6

Write a simple web server

Start by writing a simple Go web server application.

Hello world

Write a basic web server in Go that will respond to a request on port 8080 with a simple message like "hello world". The editor pane on the top right of this screen is already set up to edit a file called hello.go. You can click the button below to copy the code into that file.

package main

import (

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("hello world\n"))

    err := http.ListenAndServe(":8080", nil)
    if err != nil {
      fmt.Printf("serving: %v\n", err)

Feel free to modify the Go code to make this app your own! But if you change the port on which the server responds to requests (8080 in the example) you'll need to remember to change that as you follow the rest of the instructions in this scenario.

Compile your app

Clicking on commands like the line below, will copy and execute it in the terminal window on the bottom right of this screen.

CGO_ENABLED=0 go build -o hello hello.go

We are turning off "CGO_ENABLED" in order to build a standalone binary executable file that we'll be able to use inside a container image, without any other dependencies required.

Run your app

Run the app in the background (using the ampersand) so that you can still type into the terminal.

./hello &

Check it works

You can make a request to your application using curl:

curl localhost:8080

You should see the text message returned to you.

Host names in this Katacoda environment

We've just made the curl request to the active port (8080, if you didn't change it) on localhost. On Katacoda you can also reach active ports through a dynamically-configured host name for your environment. In this case you can access your app through your browser by visiting https://[[HOST_SUBDOMAIN]]-8080-[[KATACODA_HOST]].environments.katacoda.com/

You can also make a request using curl:

curl https://[[HOST_SUBDOMAIN]]-8080-[[KATACODA_HOST]].environments.katacoda.com/

You should also be able to issue that curl request from a terminal on your laptop, because that address is exposed to the internet.

Stop the application

When you're satisfied that this application works, let's stop it:

kill %1

Verify that hello is no longer running:


Next step

At this stage you have a compiled Go binary. In the next step we'll build a container image that includes that binary.