Docker: Building Docker Images with Dockerfiles

In this guide we’ll learn about the Dockerfile. What it is, how to create one, and how to configure the basics to bring up your own Dockerized app.

What is a Dockerfile?

  • A Dockerfile is a text configuration file written in a popular, human-readable Markup Language called YAML.
  • It is a step-by-step script of all the commands you need to run to assemble a Docker Image.
  • The docker build command processes this file generating a Docker Image in your Local Image Cache, which you can then start-up using the docker run command, or push to a permanent Image Repository.

Create a Dockerfile

Creating a Dockerfile is as easy as creating a new file named “Dockerfile” with your text editor of choice and defining some instructions.

#
# Each instruction in this file generates a new layer that gets pushed to your local image cache
#

#
# Lines preceeded by # are regarded as comments and ignored
#

#
# The line below states we will base our new image on the Latest Official Ubuntu
FROM ubuntu:latest

#
# Identify the maintainer of an image
MAINTAINER My Name “myname@somecompany.com”

#
# Update the image to the latest packages
RUN apt-get update && apt-get upgrade -y

#
# Install NGINX to test.
RUN apt-get install nginx -y

#
# Expose port 80
EXPOSE 80

#
# Last is the actual command to start up NGINX within our Container
CMD [“nginx”, “-g”, “daemon off;”]

Run below command to build image from Dockerfile

$ docker build . -t <image tag name>

eg.$ docker build . -t premaseem/dockerimage

 

Dockerfile Commands

  • ADD – Defines files to copy from the Host file system onto the Container
    • ADD ./local/config.file /etc/service/config.file
  • CMD – This is the command that will run when the Container starts
    • CMD [“nginx”, “-g”, “daemon off;”]
  • ENTRYPOINT – Sets the default application used every time a Container is created from the Image. If used in conjunction with CMD, you can remove the application and just define the arguments there
    • CMD Hello World!
    • ENTRYPOINT echo
  • ENV – Set/modify the environment variables within Containers created from the Image.
    • ENV VERSION 1.0
  • EXPOSE – Define which Container ports to expose
    • EXPOSE 80
  • FROM – Select the base image to build the new image on top of
    • FROM ubuntu:latest
  • MAINTAINER – Optional field to let you identify yourself as the maintainer of this image
    • MAINTAINER Some One “someone@xyz.xyz”
  • RUN – Specify commands to make changes to your Image and subsequently the Containers started from this Image. This includes updating packages, installing software, adding users, creating an initial database, setting up certificates, etc. These are the commands you would run at the command line to install and configure your application
    • RUN apt-get update && apt-get upgrade -y && apt-get install -y nginx && rm -rf/var/lib/apt/lists/*
  • USER – Define the default User all commands will be run as within any Container created from your Image. It can be either a UID or username
    • USER docker
  • VOLUME – Creates a mount point within the Container linking it back to file systems accessible by the Docker Host. New Volumes get populated with the pre-existing contents of the specified location in the image. It is specially relevant to mention is that defining Volumes in a Dockerfile can lead to issues. Volumes should be managed with docker-compose or “docker run” commands.
    • VOLUME /var/log
  • WORKDIR – Define the default working directory for the command defined in the “ENTRYPOINT” or “CMD” instructions
    • WORKDIR /home
Advertisements