Dockerfile

Intro

What is a Dockerfile ? Docker.com describes it like this "A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image" So it's a file, with the name Dockerfile that contains the steps, on how to build the container.

Sounds simple right ? Well that's because it is.

Below is a sample of how a simple Dockerfile could look.

Dockerfile

FROM nginx:latest
WORKDIR usr/share/nginx/html
RUN apt-get update && apt-get upgrade -y
RUN echo "test"
COPY ./html /usr/share/nginx/html
EXPOSE 80

Let's look at it, line for line.

FROM nginx:latest

Sets the source container, that we are building upon, to the latest Nginx container, from DockerHub.

WORKDIR usr/share/nginx/html

Sets the working directory, inside the container, where all commands will be run from.

RUN apt-get update && apt-get upgrade -y

Runs a simple update and upgrade, of all packages inside the container. The cammand reqires the container os, to be able to use apt as packagemanager.

RUN echo "test"

Runs another simple command.

COPY ./html /usr/share/nginx/html

Copys the files, from the source operating system, to the container.

EXPOSE 80

Let's the admin know, that this container responds to port 80. Note it's only for information.

Build

Now that we understand the Dockerfile, let's try to build a conainer from it.

Take the content of the dockerfile, and save it into a file called Dockerfile in the root directory.

To build it, we need to run the command

docker build -t container:1 .

Note the . in the end. It's important, and means that docker should look for a Dockerfile in the current directory, so don't forget it. The -t is to give the container a tag, so we can request it. A tag consist of a name:version You can use latest to get the lastest version, but it's considered best practice, to always describe the version, so you are sure what you are getting, and you are able to revert to an earlier version, if that is needed. For now, our version is 1

After a bit of time, you should see output, that looks something like this.

=> exporting to image                                   1.4s
=> => exporting layers                                  1.3s
=> => writing image sha256:feb72bf5b3f05eace0f62414347  0.0s
=> => naming to docker.io/library/container:1           0.0s

This means that you have now created a container, with the name container:1 on your local system.

To run it, you simply you simply run

docker run container:1

The container starts the Nginx process, and halts there, waiting for inputs. You can, stop it, using ctrl + c

Some containers are build to keep running, like this webserver, and others are just running some commands, and exits after. Both is valid usecases.