Pin Golang images to debian stretch variant

[thaJeztah]

the default switched to debian buster, which will have side-effects (e.g. libgcc-6-dev is no longer in buster)

[thaJeztah]

ping @seemethere @kolyshkin PTAL

[thaJeztah]

Actually; not sure if we need this, because we only copy the Go binaries 🤔

[thaJeztah]

oh, never mind; we do use the golang image for the image variant;

docker-ce-packaging/image/Dockerfile.engine

Lines 3 to 15 in 06ff8d1

	FROM ${GO_IMAGE} as builder
	COPY hack/dockerfile/install/tini.installer /
	COPY hack/dockerfile/install/proxy.installer /
	RUN apt-get update && apt-get install -y \
	bash \
	btrfs-tools \
	ca-certificates \
	cmake \
	gcc \
	git \
	libc-dev \
	libgcc-6-dev \

[seemethere]

This is a fine intermediary step but we should work to get back on the latest versions of these images eventually

[thaJeztah]

yeah, there's more side effects to switching to buster; see moby/moby#39549