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ARROW-3699: [C++] Dockerfile for testing 32-bit C++ build

I'm not sure whether this is going to work on a amd64 linux host, but it actually runs on docker for mac.

@pitrou could you please try

```
ARCH=i386 archery docker run -e ARROW_GANDIVA=OFF -e ARROW_S3=OFF debian-cpp
```

locally?

Closes #10865 from kszucs/ARROW-3699

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>