[519] Cross-arch builds: builders variables (#907)

Added PLATFORM variable to `common-envs.mk`: if not defined, it is
calculated based on docker daemon arch.
May be overridden by e.g. `make -e PLATFORM='linux/arm64' ...`
Added the variable to a single Dockerfile for now.

packages/system/cozystack-controller/Makefile

@@ -9,6 +9,8 @@ image: image-cozystack-controller update-version
 image-cozystack-controller:
 	docker buildx build -f images/cozystack-controller/Dockerfile ../../.. \
 		--provenance false \
+		--builder=$(BUILDER) \
+		--platform=$(PLATFORM) \
 		--tag $(REGISTRY)/cozystack-controller:$(call settag,$(TAG)) \
 		--cache-from type=registry,ref=$(REGISTRY)/cozystack-controller:latest \
 		--cache-to type=inline \

scripts/common-envs.mk

@@ -14,3 +14,9 @@ ifeq ($(COZYSTACK_VERSION),)
     $(shell git fetch upstream --tags)
     COZYSTACK_VERSION = $(patsubst v%,%,$(shell git describe --tags))
 endif
+
+# Get the name of the selected docker buildx builder
+BUILDER ?= $(shell docker buildx inspect --bootstrap | head -n2 | awk '/^Name:/{print $$NF}')
+# Get platforms supported by the builder
+PLATFORM ?= $(shell docker buildx ls --format=json | jq -r 'select(.Name == "$(BUILDER)") | [.Nodes[].Platforms // []] | flatten | unique | map(select(test("^linux/amd64$$|^linux/arm64$$"))) | join(",")')
+