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ARROW-7374: [Dev] [C++] Fix cuda-cpp docker build

Also add a CUDA 9.1 Dockerfile for Ubuntu 18.04 hosts.
(official Ubuntu 18.04 packages don't provide CUDA 10.0, only 9.1)

Closes #6016 from pitrou/ARROW-7374-docker-cuda-cpp and squashes the following commits:

6fa82b209 <Antoine Pitrou> Apply @kou's proposed fix
3e6597256 <Antoine Pitrou> ARROW-7374:   Fix cuda-cpp docker build

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>