rearrange for readability; add some more comments

Dockerfile

@@ -1,16 +1,26 @@
 FROM golang:1.11.10-alpine as builder
 MAINTAINER FullStory Engineering
 
+# currently, a module build requires gcc (so Go tool can build
+# module-aware versions of std library; it ships only w/ the
+# non-module versions)
 RUN apk update && apk add --no-cache ca-certificates git gcc g++ libc-dev
 # create non-privileged group and user
 RUN addgroup -S grpcurl && adduser -S grpcurl -G grpcurl
 
-# build grpcurl
 WORKDIR /tmp/fullstorydev/grpcurl
 # copy just the files/sources we need to build grpcurl
 COPY VERSION *.go go.* /tmp/fullstorydev/grpcurl/
 COPY cmd /tmp/fullstorydev/grpcurl/cmd
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 GO111MODULE=on go build -o /grpcurl -ldflags "-w -extldflags \"-static\" -X \"main.version=$(cat VERSION)\"" ./cmd/grpcurl
+# and build a completely static binary (so we can use
+# scratch as basis for the final image)
+ENV CGO_ENABLED=0
+ENV GOOS=linux
+ENV GOARCH=amd64
+ENV GO111MODULE=on
+RUN go build -o /grpcurl \
+    -ldflags "-w -extldflags \"-static\" -X \"main.version=$(cat VERSION)\"" \
+    ./cmd/grpcurl
 
 # New FROM so we have a nice'n'tiny image
 FROM scratch