update Dockerfile to use multi-stage build

docker/Dockerfile

@@ -1,20 +1,19 @@
-FROM golang:1.16.6-alpine3.14
+FROM golang:1.16.8-bullseye AS builder
 
-WORKDIR /app
+WORKDIR /src
+
+COPY go.mod go.sum ./
+RUN go mod download -x
 
-RUN apk update \
-  && apk add --virtual build-deps gcc git \
-  && rm -rf /var/cache/apk/*
+COPY . ./
 
-RUN addgroup -S golang \
-  && adduser -S -G golang golang
+RUN GOOS=linux GOARCH=amd64 go build -o /bin/courier ./cmd/courier/*.go
 
-COPY . .
+FROM gcr.io/distroless/base-debian11
 
-RUN go install -v ./cmd/...
-RUN chown -R golang /app
+WORKDIR /app
 
-USER golang
+COPY --from=builder bin/courier ./
 
 EXPOSE 8080
-ENTRYPOINT ["courier"]
+ENTRYPOINT ["./courier"]