Update container to get package.json from new path for volta

.dockerignore

@@ -1,2 +1,2 @@
 **/*
-!gui/package.json
+!desktop/package.json

building/Dockerfile

@@ -103,7 +103,7 @@ RUN apt-get update -y && \
 ENV PATH=/root/.volta/bin:$PATH
 # volta seemingly does not have a way to explicitly install the toolchain
 # versions from package.json, but `node --version` triggers an install
-COPY desktop/packages/mullvad-vpn/package.json .
+COPY desktop/package.json .
 RUN curl https://get.volta.sh | bash && node --version && rm package.json
 
 # === Golang ===