Extend docker container

Question

I am new to Docker and I have a question regarding possibility to extend docker image after pulling it from docker repository. I need specifically docker image with ubuntu 14.04, java and R. I see there are images separately with the three of them. My question is where is the Dockerfile of newly pulled images so I can extend them?

Answer 1

docker pull command just pull a pre-built image. There isn't Dockerfile on your local machine. But you can find the original Dockerfile from Docker Hub. If you use itzg/ubuntu-openjdk-7 image you can access it's online repository.

https://registry.hub.docker.com/u/itzg/ubuntu-openjdk-7/

Dockerfile is available on Dockerfile tab(Only "Automated Build" repositories have Dockefile tab on Docker Hub).

FROM ubuntu:trusty
MAINTAINER itzg
ENV APT_GET_UPDATE 2014-07-19
RUN apt-get update
RUN apt-get install -y openjdk-7-jre-headless
ENV JAVA_HOME /usr/lib/jvm/java-7-openjdk-amd64

Below is another Dockerfile from edwindj/docker-r.

FROM ubuntu:trusty
MAINTAINER Edwin de Jonge
RUN apt-get update
RUN apt-get install -y r-base

There is no method to automatically merge two or more Dockerfiles, but you can combine these two Dockerfile like below.

FROM ubuntu:trusty
RUN apt-get update

# Install java
RUN apt-get install -y openjdk-7-jre-headless
ENV JAVA_HOME /usr/lib/jvm/java-7-openjdk-amd64

# Install R
RUN apt-get install -y r-base

And build it with docker build command.

$ docker build nacyot/ubuntu-java-r .

Try to java and R command on the container which based on nacyot/ubuntu-java-r image.

$ docker run -it nacyot/ubuntu-java-r cat /etc/lsb-release 
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=14.04
DISTRIB_CODENAME=trusty
DISTRIB_DESCRIPTION="Ubuntu 14.04.1 LTS"

$ docker run -it nacyot/ubuntu-java-r java -version
java version "1.7.0_55"
OpenJDK Runtime Environment (IcedTea 2.4.7) (7u55-2.4.7-1ubuntu1)
OpenJDK 64-Bit Server VM (build 24.51-b03, mixed mode)

$ docker run -it nacyot/ubuntu-java-r R --version
R version 3.0.2 (2013-09-25) -- "Frisbee Sailing"
Copyright (C) 2013 The R Foundation for Statistical Computing
Platform: x86_64-pc-linux-gnu (64-bit)

R is free software and comes with ABSOLUTELY NO WARRANTY.
You are welcome to redistribute it under the terms of the
GNU General Public License versions 2 or 3.
For more information about these matters see
http://www.gnu.org/licenses/.

It works fine.

There is no magic, but you can extend and bulid (almost) any images which have packages you need at first hand by referencing Dockerfiles on Docker Hub.

Answer 2

Dockerfiles are made to build image but you the inverse operation isn't possible.

If you want to extend an existing image you have 2 solution.

• Run your image in a container, do the change and then commit to a new image. https://docs.docker.com/reference/commandline/cli/
• Write a Dockerfile with the image you want as base. (1st line of Dockerfile should be FROM myimage) https://docs.docker.com/reference/builder/