scripts: created Makefile.docker

diff --git a/scripts/Makefile.docker b/scripts/Makefile.docker
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+++ b/scripts/Makefile.docker
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+# Based on https://netfuture.ch/2020/05/multi-arch-docker-image-easy/
+#
+# Example: make -f Makefile.docker docker-multiarch
+
+REGISTRY       = cznic
+BASETAG                = ${REGISTRY}/knot-resolver
+PLATFORMS      = linux/amd64,linux/arm/v7,linux/arm64/v8
+
+RELEASE                = $(shell git describe --abbrev=0 --exact-match)
+RELTAG         = $(shell [ -n "${RELEASE}" ] && echo "-t ${BASETAG}:${RELEASE}" || echo "")
+
+BUILDXDETECT   = ${HOME}/.docker/cli-plugins/docker-buildx
+QEMUDETECT     = /proc/sys/fs/binfmt_misc/qemu-m68k
+
+docker-multiarch: qemu buildx docker-multiarch-builder
+       docker login
+       docker buildx build --no-cache --builder docker-multiarch --pull --push \
+                           --platform ${PLATFORMS} ${RELTAG} ..
+
+qemu: ${QEMUDETECT}
+${QEMUDETECT}:
+       docker pull multiarch/qemu-user-static
+       docker run --privileged multiarch/qemu-user-static --reset -p yes
+       docker ps -a | sed -n 's, *multiarch/qemu-user-static.*,,p' \
+         | (xargs docker rm 2>&1 || \
+           echo "Cannot remove docker container on ZFS; retry after next reboot") \
+         | grep -v 'dataset is busy'
+
+buildx: ${BUILDXDETECT}
+${BUILDXDETECT}:
+       @echo
+# Output of `uname -m` is too different
+       @echo '*** `docker buildx` missing. Install binary for this machine architecture'
+       @echo '*** from `https://github.com/docker/buildx/releases/latest`'
+       @echo '*** to `~/.docker/cli-plugins/docker-buildx` and `chmod +x` it.'
+       @echo
+       @exit 1
+
+docker-multiarch-builder: qemu buildx
+       if ! docker buildx ls | grep -w docker-multiarch > /dev/null; then \
+               docker buildx create --name docker-multiarch && \
+               docker buildx inspect --builder docker-multiarch --bootstrap; \
+       fi
+
+.PHONY: qemu buildx docker-multiarch docker-multiarch-builder
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