building packages with docker

This is a simple idea I had while waiting for a build to complete.

The background Its common to use dpkg-buildpackage inside of a chroot managed by pdebuild. The pdebuild script takes a minimal base image tarball and installs all dependencies in it required to build your package.

The build dependencies come from debian/control Build-Depends section. The package being built is a python application using virtualenv. A lot of the packages come from apt however. There is a bunch of python pip/easy install work performed too.

Needless to say this takes quite some time to do since extracting a .deb and installing it is a pretty expensive operation when there is a lot of dependencies.

Obverve the following times

• pdebuild - 2m35.113s
• dpkg-buildpackage - 0m55.480s
• docker - 0m37.421s

because i'm not interested in explaining how the other 2 method work, i'll leave that out. However, let me explain how the docker build works.

docker _

The idea is to exploit the layered file system of containers to provide build dependencies. Since its a copy on write file system, this means providing a container with all build deps present is incredibly fast.

Dockerfile

FROM ubuntu
RUN apt-get update \
   && apt-get install -y dpkg-dev


# This is the key line, it installs the build dependencies for your application!
RUN apt-get build-dep -y --force-yes YOUR_PACKAGE

RUN useradd packaging \
   && mkdir -p /packaging \
   && chown packaging /packaging

USER packaging
ADD build.sh /bin/build.sh

ENTRYPOINT [ "/bin/build.sh" ]

build.sh

#!/bin/bash

echo "[build.sh] $@"

set -x

cd /packaging
pwd
tar vzxf -
cd package
dpkg-buildpackage

Then to invoke the build we stream the contents at a tar.gz into the container which streams it to disk and builds the package.

package_stream ()
{
    git archive HEAD --format tar --prefix=package/ | gzip -9
}

Finally, this is the final command that runs the build

  time package_stream | docker run -i debbie