How Can I Manually Fetch Go Dependencies for Faster Docker Builds?

Manual Dependency Fetching in Go

One of the key concepts introduced in Go 1.11 is module support. While the automatic dependency installation feature is usually beneficial, some use cases might require manual dependency fetching.

For example, in a Docker build environment, where dependency changes are far less frequent than code changes, leveraging Docker's layer caching can significantly speed up rebuilds by separating the dependency installation stage from the build stage.

Solution

Go now provides a solution to this challenge. Thanks to issue #26610, you can utilize the go mod download command to manually fetch dependencies. This command requires only the presence of the go.mod and go.sum files.

Here's an example of a cached multistage Docker build that employs this technique:

FROM golang:1.17-alpine as builder
RUN apk --no-cache add ca-certificates git
WORKDIR /build

# Fetch dependencies
COPY go.mod go.sum ./
RUN go mod download

# Build
COPY . ./
RUN CGO_ENABLED=0 go build

# Create final image
FROM alpine
WORKDIR /
COPY --from=builder /build/myapp .
EXPOSE 8080
CMD ["./myapp"]

Additionally, the article "Containerize Your Go Developer Environment – Part 2" provides valuable insights on leveraging the Go compiler cache to further enhance build speeds.

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