chore(tests): run flatpak runtime install in local Dockerfile

Needs to be here because the command causes Docker Hub's build workers to time out.

ci/Dockerfile

@@ -1,6 +1,8 @@
 FROM malept/electron-forge-container:latest
 
-RUN apt-get install --no-install-recommends -y g++
+RUN flatpak remote-add --from gnome https://sdk.gnome.org/gnome.flatpakrepo && \
+    flatpak install --runtime gnome org.freedesktop.Platform//1.4
+
 RUN mkdir /code
 WORKDIR /code
 ADD . /code/