different approach to fetch upstream

Dockerfile

@@ -1,6 +1,5 @@
 FROM alpine:3.13 AS builder
 
-COPY tmp/unbound/ tmp/unbound
 RUN \
   apk update && \
   apk add alpine-sdk && \
@@ -9,7 +8,10 @@ RUN \
   apk add openssl-dev && \
   apk add expat-dev && \
   apk add libevent-dev && \
-  cd tmp/unbound && \
+  mkdir tmp && \
+  cd tmp && \
+  git clone https://github.com/NLnetLabs/unbound.git && \
+  cd unbound && \
   ./configure --with-libnghttp2 --with-libevent --prefix /opt/unbound && \
   make && \
   make install