chore: improve Dockerfile.

Dockerfile

@@ -1,10 +1,26 @@
-FROM golang:1.12 as builder
+FROM golang:1.13-alpine as builder
+
+RUN apk --no-cache --no-progress add git ca-certificates tzdata make \
+    && update-ca-certificates \
+    && rm -rf /var/cache/apk/*
+
 WORKDIR /go/whoami
+
+# Download go modules
+COPY go.mod .
+COPY go.sum .
+RUN GO111MODULE=on GOPROXY=https://proxy.golang.org go mod download
+
 COPY . .
+
 RUN make build
 
 # Create a minimal container to run a Golang static binary
 FROM scratch
+
+COPY --from=builder /usr/share/zoneinfo /usr/share/zoneinfo
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
 COPY --from=builder /go/whoami/whoami .
+
 ENTRYPOINT ["/whoami"]
 EXPOSE 80