Dockerfile: make image build from local source code instead of copying outside build.

Dockerfile

@@ -1,11 +1,20 @@
 FROM alpine:latest as certs
 RUN apk --update add ca-certificates
 
+FROM golang:alpine as build
+WORKDIR /app/
+
+COPY go.mod .
+COPY go.sum .
+RUN go mod download
+
+COPY . .
+RUN go build -v
+
 FROM scratch
 COPY --from=certs /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
+COPY --from=build /app/webdav /webdav
 
 EXPOSE 80
 
-COPY webdav /webdav
-
 ENTRYPOINT [ "/webdav" ]