This replaces the previous Dockerfile, which was a single stage for
building and deployment.
The new one is a multi-stage build. Its "dev" target has the full
development environment; its "deploy" target is more slim. It supports
cross-compiled builds via BuildKit, eg to prepare a build suitable for
a Raspberry Pi:
docker buildx build --load --platform=linux/arm64/v8 --tag=moonfire-nvr --progress=plain --target=deploy -f docker/Dockerfile .
Coming next: updating the installation docs.
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