There is no point in doing a different docker image for each build we
are going to do inside an Ubuntu distro. We can later use the same image
for native ubuntu builds, or other cross builds (e.g. android API 21,
etc).
It was installed in '/' which doesn't feel right. Installing it in /root
is also not correct because we want to run the build as user instead of
root in the future and cleanup.sh removes everything in /root. /opt
seems the best place because that's also the default location when
installing Android Studio.
