Docker's non-buildkit multi-arch support is pretty flaky

packaging/Makefile

@@ -82,6 +82,7 @@ build-image: check-tools .build-image.$(ARCH)
 .build-image.$(ARCH): $(KEYFILE)
 	docker build \
 		$(DOCKER_BUILD_ARGS) \
+		--pull \
 		--platform=linux/$(DOCKER_ARCH) \
 		-t synit-build:$(ARCH) .
 	touch $@
@@ -89,6 +90,7 @@ build-image: check-tools .build-image.$(ARCH)
 .build-image.rust.$(ARCH): .build-image.$(ARCH)
 	docker build \
 		$(DOCKER_BUILD_ARGS) \
+		--pull \
 		--platform=linux/$(DOCKER_ARCH) \
 		-t synit-build-rust:$(ARCH) -f Dockerfile.rust .
 	touch $@

packaging/map_docker_arch.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 case $1 in
     aarch64) echo arm64;;
-    x86_64) echo x86_64;;
+    x86_64) echo amd64;;
     armv7) echo armhf;;
     *)
         echo 'ERROR: Unknown Alpine ARCH '"$1" >&2