Docker image size optimization

Building docker images is pretty easy thanks to dockerfiles. But building small docker images is not always easy. The cache mechanism is really powerful, but it’s also the main problem when dealing with image size optimization. Let’s take an example and let’s see how to improve the dockerfile to improve the image size.

Docker

A Dockerfile as example

Base image size

As you can see we are using ubuntu 14.04 as the base image.
This base image size is 188.3 MB.
The image already has 5 layers.

More packages, elasticsearch, npm, and a PHP project (Satisfy)

The image size is now 703.4 MB
The image has 15 layers.

Optimization of the dockerfile

Packages

We do not need all recommended packages, so we can add the option --no-install-recommends to the apt-get install command.
We can clean apt and remove temporary files.

RUN apt-get update && \
  apt-get install -y --force-yes --no-install-recommends \
  build-essential \
  ...
  npm \
  && apt-get clean \
  && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

Add and remove archives or installers in the same layer

RUN curl -L -O https://download.elastic.co/elasticsearch/release/org/elasticsearch/distribution/tar/elasticsearch/2.2.0/elasticsearch-2.2.0.tar.gz && \
  tar -xvf elasticsearch-2.2.0.tar.gz && \
  rm -rf elasticsearch-2.2.0.tar.gz

Change chmod on the same layer

RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer && \
  /usr/local/bin/composer create-project playbloom/satisfy:2.0.6 --stability=dev && \
  chmod -R 777 /satisfy

Use prestissimo with Composer

Minimize image size
Drastically improve composer install runtime

RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer \
  && /usr/local/bin/composer global require hirak/prestissimo \
  && /usr/local/bin/composer create-project playbloom/satisfy:2.0.6 --stability=dev \
  && chmod -R 777 /satisfy

After all those optimizations:

$ docker images | grep img_size_optim
img_size_optim                        latest              854039a1452a        11 seconds ago      561.1 MB

The image size is now 561.1 MB
The image has 11 layers.

Remove application cache and temporary files and directories

RUN npm install express \
  && rm -rf /root/.npm/cache/*

RUN curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer \
  && /usr/local/bin/composer global require hirak/prestissimo \
  && /usr/local/bin/composer create-project playbloom/satisfy:2.0.6 --stability=dev \
  && chmod -R 777 /satisfy \
  && rm -rf /root/.composer/cache/*

$ docker images | grep img_size_optim
img_size_optim                        latest              9ad790d290db        11 seconds ago      533.7 MB

The image size is now 533.7 MB

All command in a single RUN

Be careful, because only complete RUN instruction can be cached. So if you RUN everything in a single RUN, you could (maybe) not use cache as you should.

RUN apt-get update && apt-get install -y --force-yes --no-install-recommends \
  ca-certificates \
  build-essential \
  git \
  curl \
  php5 \
  php5-mcrypt \
  php5-tidy \
  php5-cli \
  php5-common \
  php5-curl \
  php5-intl \
  php5-fpm \
  php-apc \
  nginx \
  ssh \
  npm \

  && curl -L -O https://download.elastic.co/elasticsearch/release/org/elasticsearch/distribution/tar/elasticsearch/2.2.0/elasticsearch-2.2.0.tar.gz \
  && tar -xvf elasticsearch-2.2.0.tar.gz \
  && rm -rf elasticsearch-2.2.0.tar.gz \
  
  && npm install express \
  
  && curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/local/bin --filename=composer \
  && /usr/local/bin/composer global require hirak/prestissimo \
  && /usr/local/bin/composer create-project playbloom/satisfy:2.0.6 --stability=dev \
  && chmod -R 777 /satisfy \
  
  && apt-get clean \
  && rm -rf /root/.npm/cache/* \
  && rm -rf /root/.composer/cache/* \
  && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*

$ docker images | grep img_size_optim
img_size_optim                        latest              e73bdb95e6db        7 seconds ago       533.7 MB

The image size is still 533.7 MB
The image has 8 layers.

Base image as small as possible

Have a look at those extremely small base images:

Flatten or squash docker image/containers

Flatten

http://tuhrig.de/flatten-a-docker-container-or-image/

So it is only possible to “flatten” a Docker container, not an image. So we need to start a container from an image first. Then we can export and import the container in one line:

$ docker run -it img_size_optim bash -c "exit"
$ docker ps -a | grep img_size_optim
8ec324116755        img_size_optim        "bash -c exit"           48 seconds ago      Exited (0) 48 seconds ago                              tiny_stallman
$ docker export 8ec324116755 | docker import - img_size_optim_flatten:latest
16ac86fde38629776c0734cbceae22a9da97bd303739e58c5c5261b3376e79d3

$ docker images | grep img_size_optim_flatten 
img_size_optim_flatten                latest              16ac86fde386        57 seconds ago      504.4 MB

The image size is now 504.4 MB
The image has 1 layer.

Complicated to use the cache mechanism… as it’s based on layers and RUN instructions.

Squash

https://github.com/jwilder/docker-squash

Conclusion

With all these tricks to optimize the docker image size…

Image size from 703.4 MB to 504.4 MB.
From 15 layers to 1.

And the dockerfile for the example in very simple.

Go !! Improve the size of your docker images !!

Yannick Pereira-Reis