.NET OTel AutoInstrumentation - multistage docker image build (open-telemetry#2680)

to reduce image size

Author: Kielek

autoinstrumentation/dotnet/Dockerfile

@@ -14,7 +14,7 @@
 #  - For auto-instrumentation by container injection, the Linux command cp is
 #    used and must be availabe in the image.
 
-FROM busybox
+FROM busybox as downloader
 
 ARG version
 
@@ -26,4 +26,8 @@ ADD https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/relea
 RUN unzip opentelemetry-dotnet-instrumentation-linux-glibc.zip &&\
     unzip opentelemetry-dotnet-instrumentation-linux-musl.zip "linux-musl-x64/*" -d . &&\
     rm opentelemetry-dotnet-instrumentation-linux-glibc.zip opentelemetry-dotnet-instrumentation-linux-musl.zip &&\
-    chmod -R go+r .
+    chmod -R go+r .
+
+FROM busybox
+
+COPY --from=downloader /autoinstrumentation /autoinstrumentation