How to Effectively Cache Dependencies in Docker Images for Golang Projects?

Pre-building and Caching Dependencies in Docker Images

When crafting Docker images, the need arises to cache dependencies to expedite the build process. This article tackles the issue of pre-building all required modules and caching them.

One approach involves adding a layer to cache dependencies while building the image. However, this step requires building the dependency, which can be time-consuming. An alternative method is to use the go build command.

Another option is to pre-build all dependencies listed in the go.mod file. Fortunately, Docker provides documentation that addresses this very issue. The suggested approach is to structure the Dockerfile in the following manner:

FROM --platform=${BUILDPLATFORM} docker.io/golang:1.16.7-alpine AS build
ARG TARGETOS
ARG TARGETARCH
WORKDIR /src
ENV CGO_ENABLED=0
COPY go.* .
RUN go mod download
COPY . .
RUN --mount=type=cache,target=/root/.cache/go-build \
GOOS=${TARGETOS} GOARCH=${TARGETARCH} go build -o /out/example .

FROM scratch
COPY --from=build /out/example /

Simplified for a single architecture, the relevant caching aspect looks like this:

FROM docker.io/golang:1.16.7-alpine AS build
WORKDIR /src
COPY go.* .
RUN go mod download
COPY . .
RUN --mount=type=cache,target=/root/.cache/go-build go build -o /out/example .

FROM scratch
COPY --from=build /out/example /

This mechanism mounts a cache directory on /root/.cache/go-build, which houses the go build cache. During the initial build, the cache is populated. Subsequent builds will leverage the cached files.

To activate this functionality, include DOCKER_BUILDKIT=1 when building the image using the docker build command, or use docker buildx. Tests have confirmed the efficacy of this approach.

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