ocidiet - Container Image Minimizer

Experimntal container image minimizer. Uses ldd and can possibly use strace in the future to detect the files needed by a binary. It minimizes the image size because it copies only files needed into a tar which than can be ADDed to for example busybox:uclibc image or from scratch.

Prerequisites

• Python 2.7 or Python 3+
• ldd - ldd(1)

What can be done with it?

There are two main use cases.

• building smaller images from other base images
• building images from local environment

Build small node.js app from offical node image

Using the offical node 6 image

$ docker run -ti -v $(pwd):/usr/src/app node:6 bash

Inside the node:6 container install your app

> cd /usr/src/app
> npm install
> ./ocidiet.py -t myapp.tar -b `which node` -e /etc/nsswitch.conf \
  /etc/resolv.conf myapp/node_modules myapp/package.json myapp/index.js
> exit

and finally on the host build you final app image

# docker build -t myapp .

using the following Dockerfile:

FROM busybox:uclibc

ADD myapp.tar /

ENTRYPOINT ["/usr/local/bin/node"]
CMD ["/usr/src/app/myapp/index.js"]

Now you have a node image which has roughly 37 MB + size of your app.

Create image from local environment

Another option usefull for quick shipping and sharing is to use it for creating images based on local dev environment. Usefull for compiled C/C++ projects.

Taking the hello world example

#include <stdio.h>

int main(void) {
    printf("Hello World\n");
    return 0;
}

and compiling it with gcc hello.c -o hello we now have an app which needs at least libc. If we install it to let's say /usr/local/bin we can create a container image by first creating the tar

$ ./ocidiet.py -t hello.tar -b /usr/local/bin/hello

and then using the following Dockerfile

FROM busybox:uclibc

ADD hello.tar /

ENTRYPOINT ["/usr/local/bin/hello"]

building the final image.

$ docker build -t hello .

which contains really just the busybox:uclibc files and the hello binary and correct version of libc.

If you don't need the busybox to poke around the running container just build it from scratch.

FROM scratch

ADD hello.tar /

ENTRYPOINT ["/usr/local/bin/hello"]

and save around 1.2 MB.