Dockerfile for a Crystal application

Build a space-effective docker image for a Crystal application

TL;DR: Build an application with static linking and use Alpine base image to reduce the size of the Docker image.

The building process of a Crystal app requires the installation of additional lib packages. Even the result binary is small, the docker image is significant.

Photo by Luke Stackpoole on Unsplash

A straightforward solution is to build a binary with all required libraries, a.k.a. Static Linking. It would allow skipping the installation of extra packages.

Crystal supports Static Linking, but it does not work with glibc (used in mostly all Linux distributives). To build an application with static linking, add the option --static .

So I changed the base image from Ubuntu to Alpine. Alpine uses musl instead of glibc.

Dockerfile

Here is my version of Dockerfile base on Alpine:

# Build image
FROM crystallang/crystal:1.0.0-alpine as builder

WORKDIR /app

# Cache dependencies
COPY ./shard.yml ./shard.lock /app/
RUN shards install --production -v

# Build a binary
COPY . /app/
RUN shards build --static --no-debug --release --production -v

# ===============
# Result image with one layer
FROM alpine:latest
WORKDIR /
COPY --from=builder /app/bin/medup .

ENTRYPOINT ["/medup"]

Results

DockerHub’s information about the image size (probably it shows the size of new layers):

Here are the sizes of images from my local machine:

$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
miry/medup alpine b673ccf47be5 53 minutes ago 23.1MB
miry/medup ubuntu 7d692dcd0711 7 months ago 439MB

As you see, the size of the Ubuntu version was 400MB, when the Alpine version is just 23MB.

References

https://crystal-lang.org/2020/02/02/alpine-based-docker-images.html

https://crystal-lang.org/reference/guides/static_linking.html

https://alpinelinux.org/

https://github.com/miry/medup/