Use buildx for multi-platform builds

Taylor Smockrequested to merge
gokaart/docker-library:alt-arch into master

I'd like to use some Raspberry Pi's as builders/testers, which requires that the images be built for arm in addition to amd64.

In this PR, I'm only adding arm64 and amd64 as specific platforms, since RPi3+ support arm64, and I do not anticipate a lot of older hardware being usable for builds.

This does require buildx, which is available in docker 19.03+. I may need to enable this in configuration (I don't know if the CI has this enabled).

Notes: I don't know if docker buildx create --use will be necessary (it was on Mac -- I built locally with that and CI_PROJECT_PATH="JOSM/docker-library" ./gradlew build).

Edited by Taylor Smock

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