Build go dependencies in separate Docker layer

Question

I'm trying to speed up Docker builds of my Go app. Right now, it's spending maybe 60s just building dependencies (it's a k8s controller, so there are a lot).

One very important constraint: my project depends on private GitHub repos. I do go mod vendor outside docker build, where I have creds for the repos set up.

My Dockerfile right now is roughly:

FROM golang:1.12

WORKDIR /src

COPY . .
RUN go build -mod=vendor
...

Even without having to download the deps, that build takes a while because it rebuilds several hundred packages every docker build.

What I'd like to do is something like:

FROM golang:1.12

WORKDIR /src

# these shouldn't change very often
COPY go.mod go.sum vendor ./
RUN go build -mod=vendor <all dependency packages>

COPY . .
RUN go build -mod=vendor
...

I tried parsing go.mod, but of course that lists modules, not packages. I tried go list but never managed to get a working incantation.

Answer 1

I've got a nasty hack that seems to work:

FROM golang:1.12

WORKDIR /src

COPY go.mod go.sum ./
COPY vendor/ ./vendor
RUN go build -mod=vendor $(cat deps|grep -v mypackage | grep -v internal)

COPY . .
RUN go build -mod=vendor
...

go list -f '{{join .Deps "\n"}}'  > deps
docker build .