Use buildarg to use correct parent variable

build_helpers/publish_docker.sh

@@ -17,7 +17,7 @@ else
     docker pull ${IMAGE_NAME}:${TAG}
     docker build --cache-from ${IMAGE_NAME}:${TAG} -t freqtrade:${TAG} .
 fi
-docker build --cache-from freqtrade:${TAG} -t freqtrade:${TAG_PLOT} -f docker/Dockerfile.plot .
+docker build --cache-from freqtrade:${TAG} --build-arg sourceimage=${TAG} -t freqtrade:${TAG_PLOT} -f docker/Dockerfile.plot .
 
 if [ $? -ne 0 ]; then
     echo "failed building image"

docker/Dockerfile.plot

@@ -1,4 +1,5 @@
-FROM freqtradeorg/freqtrade:develop
+ARG sourceimage=develop
+FROM freqtradeorg/freqtrade:${sourceimage}
 
 # Install dependencies
 COPY requirements-plot.txt /freqtrade/