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.
A simple Dockerfile with such characteristics would look like this:
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
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
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=linuxis 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=onbecause I like my applications using Go modules and in some older versions is not default.
- Declare a
WORKDIRso there’s no question as to where the code should be located at.
ca-certificates, because Go modules requires
gitin order to clone dependencies and
ca-certificatesso it checks the SSL/TLS certs in the connection are valid. I know I said
--no-cachebut I don’t love the extra empty directory being around my resulting images.
go.sumbefore 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
What if I could just copy the binary and run it? That’s what the second part does, so it just:
- Takes the
alpineimage as the start of it.
ca-certificatesso we can validate TLS certificates of external services.
- Copies the binary from the previous stage (note the use of
- Declares a default
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.