Dockerfile: idempotent copy

Change-Id: I5c0ef71538c1b3886787401f82ef29739948e961

Dockerfile

@@ -37,7 +37,8 @@ CMD java -jar jams-launcher.jar & npm start --prefix ../jams-react-client
 FROM build as prod
 WORKDIR /app/jams-react-client
 RUN npm run build
-RUN mv build/* ../jams-server/src/main/resources/webapp
+RUN mkdir -p ../jams-server/src/main/resources/webapp \
+    && mv build/* ../jams-server/src/main/resources/webapp
 WORKDIR /app
 RUN mvn package
 

README.md

@@ -77,7 +77,7 @@ The following commands will generate the userguide and the jars needed:
 ```
 docker build -f Dockerfile -t jams:latest --target prod . \
     && CONTAINER=$(docker create jams:latest) \
-    && docker cp $CONTAINER:/app/jams jams \
+    && docker cp $CONTAINER:/app/jams/. jams \
     && docker rm -v $CONTAINER \
     && cd jams \
     && java -jar jams-launcher.jar