docker optimization

* move openwebrx project tools to a separate layer for lower download
  volume and faster builds
* use COPY instead of ADD
* COPY multiple files at once to reduce number of layers

docker/Dockerfiles/Dockerfile-rtlsdr

@@ -1,12 +1,12 @@
 ARG ARCHTAG
 FROM openwebrx-base:$ARCHTAG
 
-ADD docker/scripts/install-dependencies-rtlsdr.sh /
-ADD docker/scripts/install-connectors.sh /
+COPY docker/scripts/install-dependencies-rtlsdr.sh \
+     docker/scripts/install-connectors.sh /
 
 RUN /install-dependencies-rtlsdr.sh &&\
     rm /install-dependencies-rtlsdr.sh &&\
     /install-connectors.sh &&\
     rm /install-connectors.sh
 
-ADD . /opt/openwebrx
+COPY . /opt/openwebrx