Reduce Docker image size of wetterdienst-full from 1.5G to 900MB

.github/release/full/Dockerfile

@@ -1,4 +1,5 @@
-FROM python:3.8.5-slim
+# 1. Build GDAL-python
+FROM python:3.8.5-slim as build-step
 
 # TODO: This currently installs GDAL==2.4.0. For a more recent version, ...
 #
@@ -24,5 +25,16 @@ RUN pip install GDAL==$(gdal-config --version)
 # Install wradlib.
 RUN pip install wradlib
 
+
+# 2. Main
+FROM python:3.8.5-slim
+
+# Install libgdal.
+RUN apt-get update
+RUN apt-get --yes install libgdal20
+
+# Copy build artefacts from first build step.
+COPY --from=build-step /usr/local/lib /usr/local/lib
+
 # Install Wetterdienst.
 RUN pip install wetterdienst[excel]