Add support for using podman if docker is unavailable

Makefile

@@ -1,4 +1,5 @@
 TIMESTAMP := $(shell date -u +"%Y%m%d%H%M%S")
+DOCKER := $(shell { command -v docker || command -v podman; })
 
 .PHONY: clean setup
 
@@ -10,11 +11,11 @@ clean:
 	rm ./firmware/*.uf2
 
 firmware/%-left.uf2 firmware/%-right.uf2: config/adv360.keymap
-	docker run --rm -it --name zmk \
+	$(DOCKER) run --rm -it --name zmk \
 		-v $(PWD)/firmware:/app/firmware \
 		-v $(PWD)/config:/app/config:ro \
 		-e TIMESTAMP=$(TIMESTAMP) \
 		zmk
 
 setup: Dockerfile bin/build.sh config/west.yml
-	docker build --tag zmk --file Dockerfile .
+	$(DOCKER) build --tag zmk --file Dockerfile .