This post will show you how to write a multi-stage Dockerfile and end up with a very small Docker image with just your application binary.

One of the very interesting things of using a compiled programming language like Go is that you end up with tiny binaries, which then you can distribute as such. If combined with Docker you can end up with very tiny images that are faster and cheaper to distribute.

TL;DR

A simple Dockerfile with such characteristics would look like this:

FROM golang:alpine AS builder

ENV CGO_ENABLED=0 \
  GOOS=linux \
  GO111MODULE=on

WORKDIR /opt/app

RUN apk --no-cache update && \
      apk --no-cache add git ca-certificates && \
      rm -rf /var/cache/apk/*

COPY go.mod .
COPY go.sum .

RUN go mod download

COPY . ./

RUN go build -a -o app .

FROM alpine

RUN apk --no-cache update && \
      apk --no-cache add ca-certificates && \
      rm -rf /var/cache/apk/*

COPY --from=builder /opt/app/app /usr/local/bin/app

CMD ["/usr/local/bin/app", "--help"]

You could copy and paste this into a Dockerfile and end up with a fairly small image and expect to call commands from /usr/local/bin/app.

What is in the Dockerfile and why

Let’s start by what’s in the first part of the Dockerfile and why it’s organized the way it is:

  • GOOS=linux is declared so it’s pretty clear and explicit what kind of OS the binary will end up being. I know it seems unnecessary and it may be it, but I find explicit better than implicit.
  • GO111MODULE=on because I like my applications using Go modules and in some older versions is not default.
  • Declare a WORKDIR so there’s no question as to where the code should be located at.
  • Install git and ca-certificates, because Go modules requires git in order to clone dependencies and ca-certificates so it checks the SSL/TLS certs in the connection are valid. I know I said --no-cache but I don’t love the extra empty directory being around my resulting images.
  • Copy go.mod and go.sum before the code, this way they can be cached separately and only if there’s a change in these files the next step will be ran.
  • go mod download, cached by the previous step.
  • Copy my source code and then build.

As the name implies, a multi-stage Dockerfile has several stages, all of which run in order during a docker build -t foo .. We ideally for a Go application would like a builder and then the final result be copied into a lightweight image. The base size for an image from golang:alpine is over 60 MB, if you’re using Go modules you’ll need git for cloning during a go mod download and ca-certificates if you want to validate the SSL certificates being used are valid. That’s already over 70MB and in most cases I end up with a binary around 8-17MB.

What if I could just copy the binary and run it? That’s what the second part does, so it just:

  • Takes the alpine image as the start of it.
  • Installs ca-certificates so we can validate TLS certificates of external services.
  • Copies the binary from the previous stage (note the use of --from=builder).
  • Declares a default CMD for --help.

Why use alpine and not scratch? Basically because most of the apps I build end up connecting to external services over TLS, and I really like to make sure the certificates are valid. Otherwise, using scratch is enough and you can decrease 5MB of the final size.