Introduction

Introduction

This guide illustrates various ways that you can improve how you build the application with Docker.

The application

The example project for this guide is a client-server application for translating messages to a fictional language.

Here’s an overview of the files included in the project:

.
├── Dockerfile
├── cmd
│   ├── client
│   │   ├── main.go
│   │   ├── request.go
│   │   └── ui.go
│   └── server
│       ├── main.go
│       └── translate.go
├── go.mod
└── go.sum

The cmd/ directory contains the code for the two application components: client and server. The client is a user interface for writing, sending, and receiving messages. The server receives messages from clients, translates them, and sends them back to the client.

The Dockerfile

A Dockerfile is a text document in which you define the build steps for your application. You write the Dockerfile in a domain-specific language, called the Dockerfile syntax.

Here’s the Dockerfile used as the starting point for this guide:

# syntax=docker/dockerfile:1
FROM golang:1.20-alpine
WORKDIR /src
COPY . .
RUN go mod download
RUN go build -o /bin/client ./cmd/client
RUN go build -o /bin/server ./cmd/server
ENTRYPOINT [ "/bin/server" ]

Here’s what this Dockerfile does:

1. # syntax=docker/dockerfile:1

   This comment is a Dockerfile parser directive. It specifies which version of the Dockerfile syntax to use. This file uses the dockerfile:1 syntax which is best practice: it ensures that you have access to the latest Docker build features.
2. FROM golang:1.20-alpine

   The FROM instruction uses version 1.20-alpine of the golang official image.

3. WORKDIR /src

   Creates the /src working directory inside the container.

4. COPY . .

   Copies the files in the build context to the working directory in the container.

5. RUN go mod download

   Downloads the necessary Go modules to the container. Go modules is the dependency management tool for the Go programming language, similar to npm install for JavaScript, or pip install for Python.

6. RUN go build -o /bin/client ./cmd/client

   Builds the client binary, used to send messages to be translated, into the /bin directory.

7. RUN go build -o /bin/server ./cmd/server

   Builds the server binary, which listens for client translation requests, into the /bin directory.

8. ENTRYPOINT [ "/bin/server" ]

   Specifies a command to run when the container starts. Starts the server process.

Build the image

To build an image using a Dockerfile, you use the docker command-line tool. The command for building an image is docker build.

Run the following command to build the image.

$ docker build --tag=buildme .

This creates an image with the tag buildme. An image tag is the name of the image.

Run the container

The image you just built contains two binaries, one for the server and one for the client. To see the translation service in action, run a container that hosts the server component, and then run another container that invokes the client.

To run a container, you use the docker run command.

1. Run a container from the image in detached mode.

   $ docker run --name=buildme --rm --detach buildme

   This starts a container named buildme.

2. Run a new command in the buildme container that invokes the client binary.

   $ docker exec -it buildme /bin/client

The docker exec command opens a terminal user interface where you can submit messages for the backend (server) process to translate.

When you’re done testing, you can stop the container:

$ docker stop buildme

Summary

This section gave you an overview of the example application used in this guide, an introduction to Dockerfiles and building. You’ve successfully built a container image and created a container from it.