What do you need to wrap a Go application in Docker? What is a multistage build and why do you copy go.mod/go.sum first, then the sources?

To containerize a Go application in Docker, you typically create a Dockerfile that describes how to build and run the application inside a container.

Multistage build is a method to create multiple build stages within a single Dockerfile. The first stage is for building the application, and the second is for creating a minimal image with the already compiled binary. This helps reduce the size of the final image by excluding build tools and source files.

Why copy go.mod and go.sum first, then the source files:

• go.mod and go.sum contain dependency information.
• By copying them separately and running go mod download, Docker caches the dependency layers.
• If the source files change but dependencies do not, Docker will not re-download dependencies, speeding up the build.

Example Dockerfile with multistage build:

# Build stage
FROM golang:1.20 AS builder
WORKDIR /app
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN go build -o myapp

# Run stage
FROM alpine:latest
WORKDIR /app
COPY --from=builder /app/myapp .
CMD ["./myapp"]

This approach allows for quick building and running of a Go application in a container with a minimal image size.