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.
docker buildcommand processes this file generating a Docker Image in your Local Image Cache, which you can then start-up using the
docker runcommand, 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.
Simple Dockerfile for NGINX
# 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
# Identify the maintainer of an image
# 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
# Last is the actual command to start up NGINX within our Container
CMD ["nginx", "-g", "daemon off;"]
- 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
- LABEL maintainer – Optional field to let you identify yourself as the maintainer of this image. This is just a label (it used to be a dedicated Docker directive).
- LABEL firstname.lastname@example.org"
- 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
Building and Testing Dockerfiles
There’s a free service that lets you quickly spin up Docker instances through a web interface called: “Play With Docker”
- First of all, head over to http://play-with-docker.com and start a new session
- Once your session is active click on “Add New Instance”:
3. A new instance will start with a Docker Engine ready to accept commands
4. Next create/edit the Dockerfile. Run “vi Dockerfile”, press “i” to switch to “Insert Mode”, copy/paste the contents of our Dockerfile, press “Esc” to exit “Insert Mode”, and save+exit by typing “:x”
5. Build the new image using the command
docker build <path>. Path refers to the directory containing the Dockerfile.
6. At the end of the process you should see the message “Successfully built <image ID>”
7. Start the new image and test connectivity to NGINX. Run the command
docker run -p 80:80 <image ID>. The option
-p 80:80 exposes the Container port 80 as the Host port 80 to the world
8. As a result a port 80 link should have become active next to the IP. Click on it to access your NGINX service
That’s all there is to it! Go have fun building your own Images!
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