Reuse go docker build cache (#1218)

cmd/fetcher/Dockerfile.fission-fetcher

@@ -1,3 +1,6 @@
+FROM alpine:3.10 as base
+RUN apk add --update ca-certificates
+
 FROM golang:1.11-alpine as builder
 RUN apk add bash ca-certificates git gcc g++ libc-dev
 
@@ -11,15 +14,28 @@ ARG BUILDDATE=unknown
 # E.g. BUILDDATE=$(date -u +'%Y-%m-%dT%H:%M:%SZ')
 
 ARG GOPKG=github.com/fission/fission
-COPY . /go/src/${GOPKG}
+
+ENV GO111MODULE=on
+
+WORKDIR /go/src/${GOPKG}
+
+# To reuse build cache, copy go.mod & go.sum and download dependencies first.
+COPY go.* ./
+
+RUN go mod download
+
+# Copy whole fission directory to work dir
+COPY ./ ./
+
 WORKDIR /go/src/${GOPKG}/cmd/fetcher
+
 RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build \
     -o /go/bin/fetcher \
     -gcflags=-trimpath=$GOPATH \
     -asmflags=-trimpath=$GOPATH \
     -ldflags "-X github.com/fission/fission/pkg/info.GitCommit=${GITCOMMIT} -X github.com/fission/fission/pkg/info.BuildDate=${BUILDDATE} -X github.com/fission/fission/pkg/info.Version=${BUILDVERSION}"
 
-FROM alpine:3.4
+FROM base
 COPY --from=builder /go/bin/fetcher /
 EXPOSE 8000