Master Docker Image Creation: From Containers to Dockerfiles

Introduction

There are three main ways to create a Docker image: using docker commit on an existing container, importing a local template with docker import, and building from a Dockerfile. This guide walks through each method with commands and verification steps.

1. Build Image from an Existing Container

The docker commit command creates a new image from a running container. Syntax:

Example steps:

root@ubuntu:~# docker run -it ubuntu:18.04 /bin/bash
root@b66c08d61f9e:/# echo "DIY" > /test
root@b66c08d61f9e:/# exit
exit

Commit the container as a new image:

root@ubuntu:~# docker commit -m "new_image" -a "cnLinuxer" b66 myubuntu:v1

The -m flag sets the commit message, -a specifies the author, b66 is the container ID, and myubuntu:v1 is the new image name.

Verify the image exists:

root@ubuntu:~# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
myubuntu            v1                  de7a0211f27f        8 minutes ago       64.2MB

Run a container from the new image to confirm the file persists:

root@ubuntu:~# docker run -it myubuntu:v1 /bin/bash
root@d11d9577f31b:/# cat /test
DIY

Building an image from a container essentially packages the running program and its environment into a reusable image.

2. Build Image by Importing a Local Template

The docker import command creates an image from a tar archive exported from a container.

Export a container to a tar file:

root@ubuntu:~# docker export b66 > /root/myubuntu.tar
root@ubuntu:~# ls /root
myubuntu.tar

Import the tar file as a new image:

root@ubuntu:~# docker import /root/myubuntu.tar qinlulu/ubuntu:18.04

Verify the imported image:

root@ubuntu:~# docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
qinlulu/ubuntu      18.04               498afccbfb2d        6 seconds ago       64.2MB

3. Build Image from a Dockerfile

Using a Dockerfile is the most common method. Each instruction in the file corresponds to a Docker command.

Example: building an Nginx image.

Prepare a directory with the Dockerfile and the Nginx source package:

root@ubuntu:~# mkdir -p /opt/nginx
root@ubuntu:~# wget -P /opt/nginx http://nginx.org/download/nginx-1.11.1.tar.gz
root@ubuntu:~# touch /opt/nginx/Dockerfile
root@ubuntu:~# ls /opt/nginx/
Dockerfile  nginx-1.11.1.tar.gz

Create the Dockerfile:

FROM centos:7.5.1804
MAINTAINER mynginx
ADD nginx-1.11.1.tar.gz /usr/local
RUN yum -y install vim wget gcc gcc-c++ make openssl-devel pcre-devel
WORKDIR /usr/local/nginx-1.11.1/
RUN ./configure --prefix=/usr/local/nginx && make && make install
RUN echo "daemon off;" >> /usr/local/nginx/conf/nginx.conf
ENV PATH /usr/local/nginx/sbin:$PATH
EXPOSE 80
CMD ["nginx"]

Build the image:

root@ubuntu:/opt/nginx# docker build -t mynginx:v1

Run the newly built Nginx container:

root@ubuntu:~# docker run -itd -p 888:80 mynginx:v1

Access the service via a browser to verify it works.