Added Dockerfile for arm32v7 (builds Docker image for Raspberry Pi 3)

[wabuMike]

This PR adds a Dockerfile compatible with arm32v7 instruction sets, specifically Raspberry Pi 3.

[jbarlow83]

Thanks for the contribution. Did you build it on Docker Hub? I'd like to see that it builds.

Also, do you know how to set up a test procedure on Docker Hub for it?

[wabuMike]

@jbarlow83 No, it's not on Docker Hub yet. I'd love to have it tested somehow. Give me a week or two to find out.

[mawi12345]

I would not maintain this as part of this repo (see #426). Maybe we could collect all other "none official / tested" Dockerfiles in a different repo?

[wabuMike]

Building images on Docker Hub for architectures different from amd64 and i386 does not work by default.

Building the proposed Dockerfile can currently only be accomplished on a CPU with arm architecture.

However, there seems to be a way with qemu (see docker/hub-feedback#1261). This will need more investigation on my side.

In the meantime, you might want to consider creating another repo like @mawi12345 suggested.

[jbarlow83]

Have a look at the recent improvements I made to .docker/Dockerfile. I eliminated some dependencies and such. These should be reflected over.

Generally I am leaning towards the idea of instead linking to the image as a solution for RaspPi users and relying on you to maintain it.

[wabuMike]

I can't find way to autobuild a Dockerfile for arm architecture in Docker Hub. Thus, I will drop this pull request.

Eventually, I might upload the arm image to my Docker Hub account directly without autobuild.