Docker Build Cacher

This tool is intended to speedup multi-stage Dockerfile build times by caching the results of each of the stages separately.

Why?

Multi-stage docker file builds are great, but they still miss a key feature: It is not possible to carry from one build to another the statically generated cache files once the source file in your project change. Here’s an example that illustrates the issue:

Imagine you create a generic Dockerfile for building node projects

FROM nodejs

RUN apt-get install nodejs yarn

WORKDIR /app

# Whenever this image is used execute these triggers
ONBUILD ADD package.json yarn.lock .
ONBUILD RUN yarn
ONBUILD RUN yarn run dist

And then you call

docker build -t nodejs-build .

So now you can use the nodejs-build image in other builds, like this:

# Automatically build yarn dependencies
FROM nodejs-build as nodedeps

# Build the final container image
FROM scratch

# Copy the generated app.js from yarn run dist
COPY --from=nodedeps /app/app.js .
...

So far so good, we have build a pretty lean docker image that discards all the node_modules folder and only keeps the final artifact. For example a bundled reactjs application.

It’s also very fast to build! Since each of the steps in the Dockerfile are cached, as long as none of the files changed.

But that’s also where the problem is: Whenever package.json or yarn.lock files change, docker will trash all the files in node_modules and all the cached yarn packages and will start from scratch downloading, linking and building every single dependency.

That’s far from ideal. What if we could do a change in the process so that changes to those files do not bust the yarn cache? It turns out that we can!

Enter docker-build-cacher

This utility overcomes the problem by providing a way to build the docker file and then cache the intermediate stages. On subsequent builds, it will make sure that the static cache files generated during previous builds will also be present.

The effect it has should be obvious: your builds will be consistently fast, at the cost of more disk space.

Installation

There are binaries provided for linux-x86_64 and MacOS, check the releases page for downloads.

Usage

docker-build-cacher requires the following environment variables to be present in order to correctly build your Dockerfile:

• APP_NAME: The name for application you are trying to build. Usually this is just the folder name you are in.
• GIT_BRANCH: The name of the git branch you are building. Used to “namespace” cache results
• DOCKER_TAG: It will docker build -t $DOCKER_TAG . at some point. Let it know the image tag you want at the end.

This utility has two modes, Build and Cache. Both modes should be invoked for the cache to work:

export APP_NAME=fancyapp
export GIT_BRANCH=master
export DOCKER_TAG=fancyapp:latest

docker-build-cacher build # This will build the docker file
docker-build-cacher cache # This will cache each of the stage results separately

Additionally, docker-build-cacher accepts the DOCKERFILE env variable in case the file is not present in the current directory:

DOCKERFILE=buildfiles/Dockerfile docker-build-cacher build

At the end of the process you can call docker images and see that it has created fancyapp:latest, and if you are using multi-stage builds, it should have created an image tag for each of the stages in your Dockerfile

How It Works

This works by parsing the Dockerfile and extracting the COPY or ADD instructions nested inside ONBUILD for each of the stages found in the file.

It will compare the source files present in such COPY or ADD instructions to check for changes. If it can detect changes, it rewrites your Dockerfile on the fly so that the FROM directives in each of the stages use the locally cached images instead of the original base image.

The effect this FROM swap has, is that disk state for the image is preserved between builds.

Passing extra arguments to docker build

It is possible to pass extra arguments and flags to the docker build step by providing the environment variable DOCKER_BUILD_OPTIONS as shown below:

DOCKER_BUILD_OPTIONS="--build-arg foo=bar --quiet" docker-build-cacher build

Building from source

Dependencies:

• Haskell stack

Install the stack tool from the link above. Then cd to the root folder of this repo and execute:

stack setup
stack install

If it is the first time, it will take a lot of time. Don’t worry, it’s only once you need to pay this price.