feat: Add multi-architecture Docker build support and push build Docker

[sanjay7178]

fixes : #23

Improvements

Dockerfile

• Added ARG TARGETOS and ARG TARGETARCH build arguments for cross-compilation
• Modified Go build command to use platform-specific compilation: CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH}
• Simplified the pkg/imports builds in dockerfile with go mod download

docker-push-multiarch.yaml

• Added .github/workflows/docker-push-multiarch.yaml for building and pushing multi-architecture images to docker hub

ci.yml

• Added docker-multiarch-build job to validate multi-arch builds on every PR and workflows support linux/amd64 and linux/arm64 platforms using Docker Buildx

Testing

I ensured this pr was tested via this github actions run for multiarch build https://github.com/sanjay7178/cmd-nsmgr/actions/runs/16680645623