imports-gen is a small Go code generator designed to 'prime' a docker container in terms of both source code download and binary caching.

Use

Simply put a gen.go file in any package in your module containing a go:generate directive:

pkg/imports/gen.go:

//go:generate bash -c "cd $(mktemp -d) && GO111MODULE=on go get github.com/edwarnicke/imports-gen@v1.1.2"
//go:generate bash -c "GOOS=linux ${GOPATH}/bin/imports-gen"

Run:

go generate ./pkg/imports

This will result in the creation of a file imports_{{GOOS}}.go, which imports every package imported anywhere in the module for that GOOS.

A docker container can then be primed with:

COPY go.mod go.sum ./
COPY ./pkg/imports/ ./pkg/imports/
RUN go build ./pkg/imports/

The layer from the 'RUN' line will have all of the dependencies, both source and binary for the go module. The rest of the source code can then be copied in and built:

COPY go.mod go.sum ./
COPY ./pkg/imports/ ./pkg/imports/
RUN go build ./pkg/imports/ # This line is cached
COPY . .
RUN go install ./...