Fix grass-session installation in alpine docker (#516)

* fix grass-session installation in alpine docker

* use smart pip command for grass-session in alpine docker

docker/alpine/Dockerfile_alpine

@@ -192,7 +192,9 @@ COPY --from=build /usr/local/bin/grass /usr/local/bin/grass
 COPY --from=build /usr/local/grass* /usr/local/grass/
 # pip 20.0.0 fix
 RUN apk add curl && curl https://bootstrap.pypa.io/get-pip.py -o get-pip.py && python3 get-pip.py pip==20.0.2 && rm get-pip.py
-RUN pip3 install --upgrade pip six grass-session
+#RUN pip3 install --upgrade pip six grass-session
+RUN apk add git
+RUN pip3 install --upgrade pip six git+git://github.com/zarch/grass-session.git@0b8414c1
 RUN ln -s /usr/local/grass /usr/local/grass7
 RUN ln -s /usr/local/grass `grass --config path`
 RUN grass --tmp-location EPSG:4326 --exec g.version -rge && \