Construct Dockerfile

Overview

Based on most of the previous steps, we are ready to build our Dockerfile. We will mimic those steps for automating our image construction.

Review

In preparation of writing our Dockerfile, let's review all the steps we've performed up to this point.

1. Install the latest version of CentOS
2. Install the latest packages
3. Install and configure Docker
4. Add a reference to EPEL
5. Install Node.js
6. Install Node Package Manager (NPM)
7. Install Git
8. Download (clone) the sample website
9. Download website dependencies
10. Run the web server

As a reminder, since we are constructing an image, we can ignore step 3. We won't need Docker installed inside of the image.

Create the Dockerfile

Let's go ahead and create the Dockerfile contents. We'll then examine each line below.

1. Return to your home folder by typing at the terminal prompt, cd ~.

2. Create a Dockerfile using the nano text editor by typing the following: nano Dockerfile. Nano is reminiscent of the old DOS editor. Of course, you can you vim instead if you are comfortable in doing so.
   NOTE 'Dockerfile' is case-sensitive.

3. Enter the following without the line numbers. The line numbers are provided for reference below.

1   FROM centos:latest
2   MAINTAINER Your Name <you@yourcompany.com>
3
4   RUN yum update -y
5   RUN yum install -y epel-release
6   RUN yum install -y nodejs
7   RUN yum install -y npm
8   RUN yum install -y git
9
10  RUN git clone https://github.com/AzureWorkshops/samples-simple-nodejs-website.git /var/www
11  WORKDIR /var/www
12  RUN npm i
13
14  EXPOSE 8080
15
16  CMD node /var/www/index.js

4. To save, Ctrl+O

5. To exit, Ctrl+X

Explanation

First, if you remember from the previous steps, we prepended each command with sudo to allow the command to be executed with elevated privileges. By default, all Docker images execute under the identity of the built-in superuser account root. Therefore, we can omit the sudo.

Line 1: Specifies the base image, including the tag, with which we're starting. In our case, we are using the minimal CentOS OS as the base image.

Line 2: Specifies the owner of the image with their email address.

Lines 4-8: The commands we executed earlier in this workshop that update the system and installs Node.js and Git.

Line 10: Downloads (clones) the sample website into the /var/www local folder.

Line 11: WORKDIR is how you change the current directory (compared to cd) in a Dockerfile. We are changing to the website home directory.

Line 12: Install the website's dependencies.

Line 14: Our website server is programmed to use port 8080. Therefore, similar to a firewall in the image, we open, or expose, the port to the outside host. We will bind to this open port later when we run a container based on this image.

Line 16: This starts our web server. We could have used the RUN directive, but the CMD directive is designed to execute our long-running process. While, technically, we could have multiple CMD lines in the Dockerfile, the Docker build will ignore all CMD lines except the last one.

That's it! That's all there is to creating a Dockerfile.