Distroless or scratch for Go apps?

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When I create docker images for a Go application, I often use the scratch image. Scratch is an empty image, so it is ideal for statically linked binaries that do not require libc. Go, Rust and other languages can compile to such binaries.

Because I like the pattern of building the binary in a container, I prefer a multi-stage build . An example of a Dockerfile for such a build can be found here and below (with some comments removed to make it shorter).

ARG GO_VERSION=1.15.6

# STAGE 1: building the executable
FROM golang:${GO_VERSION}-alpine AS build
RUN apk add --no-cache git
RUN apk --no-cache add ca-certificates

# add a user here because addgroup and adduser are not available in scratch
RUN addgroup -S myapp \
    && adduser -S -u 10000 -g myapp myapp

WORKDIR /src
COPY ./go.mod ./go.sum ./
RUN go mod download

COPY ./ ./

# Run tests
RUN CGO_ENABLED=0 go test -timeout 30s -v github.com/gbaeke/go-template/pkg/api

# Build the executable
RUN CGO_ENABLED=0 go build \
	-installsuffix 'static' \
	-o /app ./cmd/app

# STAGE 2: build the container to run
FROM scratch AS final
LABEL maintainer="gbaeke"
COPY --from=build /app /app

# copy ca certs
COPY --from=build /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/

# copy users from builder (use from=0 for illustration purposes)
COPY --from=0 /etc/passwd /etc/passwd

USER myapp

ENTRYPOINT ["/app"]

Because the scratch image is empty, there are several things missing:

  • CA (certificate authority) certificates
  • /etc/passwd with users and groups

In order to add the certificates, they are installed in the first stage and later copied. Similarly, if you want to create explicit users and groups and use them in the scratch image, you create them in the first stage and copy /etc/passwd in the later stage. Instead of copying /etc/passwd, you can also skip that and just set the USER with a uid like USER 10000. I all comes down to what you need exactly or how explicit you want to be.

Although this works, you can also use distroless static. The distroless static image is over at grc.io, at gcr.io/distroless/static. This image is ideal for Go apps that do not need libc. It is not completely empty and includes the following:

  • CA certs: no need to copy them from stage 1
  • /etc/passwd: contains users and groups such as nonroot
  • /tmp
  • tzdata: in case you want to set the timezone other than UTC

Creating a Docker image based on distroless static is easy to do:

# argument for Go version
ARG GO_VERSION=1.15.6

# STAGE 1: building the executable
FROM golang:${GO_VERSION}-alpine AS build

RUN apk add --no-cache git
WORKDIR /src
COPY ./go.mod ./go.sum ./
RUN go mod download
COPY ./ ./
# Run tests
RUN CGO_ENABLED=0 go test -timeout 30s -v github.com/gbaeke/go-template/pkg/api
# Build the executable
RUN CGO_ENABLED=0 go build \
	-installsuffix 'static' \
	-o /app ./cmd/app

# STAGE 2: build the container to run
FROM gcr.io/distroless/static AS final

LABEL maintainer="gbaeke"
USER nonroot:nonroot

# copy compiled app
COPY --from=build --chown=nonroot:nonroot /app /app

# run binary; use vector form
ENTRYPOINT ["/app"]

In the above Dockerfile, there is no need to copy CA certs or etc/passwd in stage 2. The CA certs are already in the image and /etc/passwd contains a nonroot user and group. Although you can set the user and group in your Kubernetes manifest, I find it a good practice to also add it to the Dockerfile.

The size of the above distroless image is around 30MB. When I use the scratch image, the size is around 29MB. Hardly a difference which makes total sense.

So what should you do? I would recommend to look at distroless images in general. They are available in several flavors and it’s certainly the right trend. Before you do, be sure to check this informative post as well.

When it comes to Go and the scratch image, I prefer using distroless static. The difference with scratch is of course very small but the inclusion of CA certs and some default users make things a tiny bit easier.

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