feat: 'make multi' to build and push multiarch docker image with arm64,amd64,arm archs

[muravjov]

Hi, thanks for your work.

I use arm64 based k8s and following the tutorial at https://prometheus-operator.dev/docs/developer/getting-started/#deploying-a-sample-application

I got an issue that the image in question cannot be run on my node because:

$ kubectl logs deployments/example-app --all-pods --prefix -f
[pod/example-app-5ffc85cf6d-96r92/example-app] exec /bin/prometheus-example-app: exec format error
[pod/example-app-5ffc85cf6d-98r8x/example-app] exec /bin/prometheus-example-app: exec format error
[pod/example-app-5ffc85cf6d-vkv96/example-app] exec /bin/prometheus-example-app: exec format error

This patch intoduces a new Makefile multi which builds and pushes a multiarch image to quay.io/brancz/prometheus-example-app:v0.5.0 .
If you want to test it locally without pushing, run make multi with this patch:

$ git diff
diff --git a/Makefile b/Makefile
index e2970a9..a2ecd85 100644
--- a/Makefile
+++ b/Makefile
@@ -9,4 +9,4 @@ all: build
        docker build -t quay.io/brancz/prometheus-example-app:$(VERSION) .

 multi: build
-       docker buildx build --push --platform linux/arm/v7,linux/arm64/v8,linux/amd64 -t quay.io/brancz/prometheus-example-app:$(VERSION) .
+       docker buildx build --platform linux/arm/v7,linux/arm64/v8,linux/amd64 -t quay.io/brancz/prometheus-example-app:$(VERSION) .