How to extend an existing docker image?

Question

I'm using the official elasticsearch Docker image instead of setting up my own elastic search instance. And that works great, up to the point when I wanted to extend it. I wanted to install marvel into that ElasticSearch instance to get more information.

Now dockerfile/elasticsearch automatically runs ElasticSearch and setting the command to /bin/bash doesn't work, neither does attaching to the container or trying to access it over SSH, nor installing ssh-daemon with apt-get install -y openssh-server.

In this particular case, I could just go into the container's file system and execute opt/elasticsearch/bint/plugin -i elasticsearch/marvel/latest and everything worked.

But how could I install an additional service which needs to be installed with apt-get when I can't have a terminal inside the running container?

Answer 1

Simply extend it using a Dockerfile that start with

FROM dockerfile/elasticsearch 

and install marvel or ssh-server or whatever you need. Then, end with the correct command to start your services. You can use supervisor to start multple services, see Run a service automatically in a docker container for more info on that.

Answer 2

If you don't mind using docker-compose, what I usually do is to add a first section for the base image you plan to reuse, and then use that image as the base in the rest of the services' Dockerfiles, something along the lines of:

--- version: '2' services:     base:         build: ./images/base      collector:          build: ./images/collector 

Then, in images/collector/Dockerfile, and since my project is called webtrack, I'd type

FROM webtrack_base ... 

And now it's done!