Cleaner multi-arch build

Create the multi-arch manifest in separate steps:
1. Build the specific tagged arch images
2. Create the manifest
3. Add the tagged image to the manifest

This makes the output much easier to manage. The time to create seems
similar to the one command build.

Example output:

    % podman image ls | grep gather
    quay.io/nirsof/gather     0.5.1-16-g928edb2-arm64  7299e9024f63  About a minute ago  110 MB
    quay.io/nirsof/gather     0.5.1-16-g928edb2        53bbc49a60ad  About a minute ago  1.06 kB
    quay.io/nirsof/gather     0.5.1-16-g928edb2-amd64  50d554dccdd8  2 minutes ago       110 MB

Makefile

@@ -26,10 +26,15 @@ ldflags := -s -w \
 
 all: kubectl-gather
 
-container:
+container: container-amd64 container-arm64
+	podman manifest create --amend $(image)
+	podman manifest add $(image) $(image)-amd64
+	podman manifest add $(image) $(image)-arm64
+
+container-%:
 	podman build \
-		--platform=linux/amd64,linux/arm64 \
-		--manifest $(image) \
+		--platform=linux/$* \
+		--tag $(image)-$* \
 		--build-arg ldflags="$(ldflags)" \
 		.