Creating new Docker images
Overview
Teaching: 15 min
Exercises: 15 minQuestions
How can you create your own Docker image?
Objectives
Create a new Docker image.
The previous lesson showed how to run existing docker images. This lesson will demonstrate how to build your own images.
What is a Dockerfile?
A Dockerfile is a text file containing a set of instructions for reproducibly building a Docker image
My first Dockerfile
Create a new directory, enter it, and create a file called Dockerfile:
mkdir my-first-dockerfile
cd my-first-dockerfile
<edit> Dockerfile e.g. use vi Dockerfile
FROM rockylinux:9
LABEL maintainer="OME"
RUN dnf install -y -q epel-release
RUN dnf install -y -q python-pip
RUN dnf install -y python3
FROM: The name of a base imageMAINTAINER: The email of the developer or ownerRUN: Runs a shell command
Build it. -t let’s you give the image a name
docker build -t my-docker-image .
Building 17.7s (8/8) FINISHED
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 199B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [internal] load metadata for docker.io/library/rockylinux:9 0.0s
=> [1/4] FROM docker.io/library/rockylinux:9 0.0s
=> [2/4] RUN dnf install -y -q epel-release 5.0s
=> [3/4] RUN dnf install -y -q python-pip 10.4s
=> [4/4] RUN pip install omego 1.9s
=> exporting to image 0.3s
=> => exporting layers 0.3s
=> => writing image sha256:5655f61f4aa8a546b788b9d34a3f48238e5f9cf4b2d7eb976aceffd0bd406878 0.0s
=> => naming to docker.io/library/my-docker-image
And run
docker run -it my-docker-image omego version
b'0.7.0'
Now build it again:
docker build -t my-docker-image .
Building 0.1s (8/8) FINISHED
=> [internal] load build definition from Dockerfile 0.0s
=> => transferring dockerfile: 34B 0.0s
=> [internal] load .dockerignore 0.0s
=> => transferring context: 2B 0.0s
=> [internal] load metadata for docker.io/library/rockylinux:9 0.0s
=> [1/4] FROM docker.io/library/rockylinux:9 0.0s
=> CACHED [2/4] RUN dnf install -y -q epel-release 0.0s
=> CACHED [3/4] RUN dnf install -y -q python-pip 0.0s
=> CACHED [4/4] RUN pip install omego 0.0s
=> exporting to image 0.0s
=> => exporting layers 0.0s
=> => writing image sha256:5655f61f4aa8a546b788b9d34a3f48238e5f9cf4b2d7eb976aceffd0bd406878 0.0s
=> => naming to docker.io/library/my-docker-image
The build is much quicker, and you may have noticed the => CACHED lines.
More Dockerfile commands
The full list of Dockerfile commands is in the Docker builder documentation. Some of the most common ones are:
COPY: Copies a file (e.g. a script, configuration file, or archive) into the Docker imageUSER: Change the user that a command is run as (useful for dropping privileges)WORKDIR: Change the current working directoryEXPOSE: Lists ports that should be exposed to the outside worldVOLUMES: Directories that should be managed separately from the container (e.g. persistent data that should be kept after the container exits)
Key Points
A Dockerfile is a set of instructions for building a Docker image
Convert a Dockerfile into an image using
docker build
docker buildhas a built-in cache to greatly speed up builds