contrib: Dockerfile: add a base stage

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

contrib/Dockerfile.test

@@ -8,8 +8,12 @@
 
 ARG GOLANG_VERSION=1.11
 
+FROM golang:${GOLANG_VERSION} AS golang-base
+RUN mkdir -p /go/src/github.com/containerd/containerd
+WORKDIR /go/src/github.com/containerd/containerd
+
 # Install proto3
-FROM golang:${GOLANG_VERSION} AS proto3
+FROM golang-base AS proto3
 RUN apt-get update && apt-get install -y \
    autoconf \
    automake \
@@ -22,17 +26,17 @@ COPY script/setup/install-protobuf install-protobuf
 RUN ./install-protobuf
 
 # Install runc
-FROM golang:${GOLANG_VERSION} AS runc
+FROM golang-base AS runc
 RUN apt-get update && apt-get install -y \
     curl \
     libseccomp-dev \
   --no-install-recommends
 
-COPY vendor.conf /go/src/github.com/containerd/containerd/vendor.conf
+COPY vendor.conf vendor.conf
 COPY script/setup/install-runc install-runc
 RUN ./install-runc
 
-FROM golang:${GOLANG_VERSION} AS dev
+FROM golang-base AS dev
 RUN apt-get update && apt-get install -y \
     btrfs-tools \
     gcc \
@@ -46,6 +50,4 @@ COPY --from=proto3 /usr/local/bin/protoc     /usr/local/bin/protoc
 COPY --from=proto3 /usr/local/include/google /usr/local/include/google
 COPY --from=runc   /usr/local/sbin/runc      /usr/local/go/bin/runc
 
-COPY . /go/src/github.com/containerd/containerd
-
-WORKDIR /go/src/github.com/containerd/containerd
+COPY . .