optimize Dockerfile for size

Dockerfile

@@ -1,16 +1,16 @@
 FROM golang:1.14 AS builder
 COPY . /app
 WORKDIR /app
+ARG GOARCH=amd64
 RUN go generate
 RUN \
     CGO_ENABLED=0 \
     GOOS=linux \
     GOPROXY=https://proxy.golang.org,direct \
-    go build -o main main.go
+    go build -ldflags "-s -w" -o main .
 
 FROM scratch
-COPY contrib/etc_group /etc/group
-COPY contrib/etc_passwd /etc/passwd
+COPY contrib/etc/group contrib/etc/passwd /etc/
 COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
 COPY --from=builder /app/main /main
 USER user