Building and Running a Docker Container

• Build a Docker Image
• Running a Docker Container
• Example Go Application

Build a Docker Image

This section explains how to create a Docker image.

Dockerfile

Docker build images by reading instructions from a Dockerfile. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. docker image build command uses this file and executes all the commands in succession to create an image.

build command is also passed a context that is used during image creation. This context can be a path on your local filesystem or a URL to a Git repository.

Dockerfile is usually called Dockerfile. The complete list of commands that can be specified in this file are explained at https://docs.docker.com/reference/builder/. The common commands are listed below:

Common commands for Dockerfile

Create your first image

Create a new directory hellodocker.

In that directory, create a new text file Dockerfile. Use the following contents:

FROM ubuntu:latest

CMD ["/bin/echo", "hello world"]

This image uses ubuntu as the base image. CMD command defines the command that needs to run. It provides a different entry point of /bin/echo and an argument “hello world”.

Build the image

  docker image build . -t helloworld

. in this command is the context for the command docker image build. -t adds a tag to the image.

The following output is shown:

Sending build context to Docker daemon  2.048kB
Step 1/2 : FROM ubuntu:latest
latest: Pulling from library/ubuntu
9fb6c798fa41: Pull complete 
3b61febd4aef: Pull complete 
9d99b9777eb0: Pull complete 
d010c8cf75d7: Pull complete 
7fac07fb303e: Pull complete 
Digest: sha256:31371c117d65387be2640b8254464102c36c4e23d2abe1f6f4667e47716483f1
Status: Downloaded newer image for ubuntu:latest
 ---> 2d696327ab2e
Step 2/2 : CMD /bin/echo hello world
 ---> Running in 9356a508590c
 ---> e61f88f3a0f7
Removing intermediate container 9356a508590c
Successfully built e61f88f3a0f7
Successfully tagged helloworld:latest

List the images

You can list the images available using docker image ls:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
helloworld          latest              e61f88f3a0f7        3 minutes ago       122MB
ubuntu              latest              2d696327ab2e        4 days ago          122MB

Other images may be shown as well but we are interested in these two images for now.

Run the container using the command:

docker container run helloworld

to see the output:

hello world

If you do not see the expected output, check your Dockerfile that the content exactly matches as shown above. Build the image again and now run it.

Change the base image from ubuntu to busybox in Dockerfile. Build the image again:

docker image build -t helloworld:2 .

and view the images using docker image ls command:

REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
helloworld          2                   7fbedda27c66        3 seconds ago       1.13MB
helloworld          latest              e61f88f3a0f7        5 minutes ago       122MB
ubuntu              latest              2d696327ab2e        4 days ago          122MB
busybox             latest              54511612f1c4        9 days ago          1.13MB

helloworld:2 is the format that allows to specify the image name and assign a tag/version to it separated by :.

Example Go Application

Create a main.go file with the following content:

Now that we have our server, let’s set about writing our Dockerfile and construct a container in which our Go application will live.

Create Dockerfile with following content:

Now that we have defined everything we need for our Go application to run in our Dockerfile we can now build an image using this file. In order to do that, we’ll need to run the following command:

$ docker build -t my-go-app .
Sending build context to Docker daemon   5.12kB
Step 1/6 : FROM golang:1.12.0-alpine3.9
 ---> d4953956cf1e
Step 2/6 : RUN mkdir /app
 ---> Using cache
 ---> be346f9ff24f
Step 3/6 : ADD . /app
 ---> eb420da7413c
Step 4/6 : WORKDIR /app
 ---> Running in d623a88e4a00
Removing intermediate container d623a88e4a00
 ---> ffc439c5bec5
Step 5/6 : RUN go build -o main .
 ---> Running in 15805f4f7685
Removing intermediate container 15805f4f7685
 ---> 31828faf8ae4
Step 6/6 : CMD ["/app/main"]
 ---> Running in 9d54463b7e84
Removing intermediate container 9d54463b7e84
 ---> 3f9244a1a240
Successfully built 3f9244a1a240
Successfully tagged my-go-app:latest

We can now verify that our image exists on our machine by using docker images command:

$ docker images
REPOSITORY                                 TAG                 IMAGE ID            CREATED             SIZE
my-go-app                                  latest              3f9244a1a240        2 minutes ago       355MB$ docker images

In order to run this newly created image, we can use the docker run command and pass in the ports we want to map to and the image we wish to run.

$ docker run -p 8080:8081 -it my-go-app

• -p 8080:8081 - This exposes our application which is running on port 8081 within our container on http://localhost:8080 on our local machine.
• -it - This flag specifies that we want to run this image in interactive mode with a tty for this container process.
• my-go-app - This is the name of the image that we want to run in a container.

Awesome! Now if you go to http://localhost:8080 in your browser, you should see that the application is responds with Hello, "/".

Run Container in Background

You’ll notice that if we ctrl-c this within the terminal, it will kill the container. If we want to have it run permanently in the background, you can replace -it with -d to run this container in detached mode.

In order to view the list of containers running in the background you can use docker ps which should output something like this:

$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                    NAMES
70fcc9195865        my-go-app           "/app/main"              5 seconds ago       Up 3 seconds        0.0.0.0:8080->8081/tcp   silly_swirles

Go Modules

Let’s look at a more complex example which features imported modules. In this instance, we will need to add a step within our Dockerfile which does the job of downloading our dependencies prior to the go build command executing: