update dockerefile for build with go 1.19 and use multistage build

docker/Dockerfile

@@ -1,21 +1,25 @@
-FROM golang:1.18.9-alpine3.17
+FROM golang:1.19-alpine3.18 AS builder
 
 WORKDIR /app
 
 RUN apk update \
   && apk add --virtual build-deps gcc git curl tar \
   && rm -rf /var/cache/apk/*
 
-RUN addgroup -S golang \
-  && adduser -S -G golang golang
-
 COPY . .
 
 RUN curl -L https://github.com/nyaruka/goflow/releases/download/v$(grep goflow go.mod | cut -d" " -f2 | cut -c2-)/docs.tar.gz --output docs.tar.gz && \
     tar -xf docs.tar.gz && rm docs.tar.gz
 
+RUN --mount=type=cache,target=/go/pkg/mod/ \
+  go install -v ./cmd/...
+
+FROM alpine:3.18
 
-RUN go install -v ./cmd/...
+COPY --from=builder /go/bin/ /app/
+COPY --from=builder /app/docs /app/docs
+
+WORKDIR /app
 
 EXPOSE 8000
-ENTRYPOINT ["mailroom"]
+ENTRYPOINT ["./mailroom"]