Make Docker registry and container tool configurable

[Zerpet]

Summary

This PR makes the Docker build process more flexible by:

• Making the Docker registry URL configurable via a DOCKER_REGISTRY build argument
• Making the container tool configurable (supporting both docker and podman)

Changes

Dockerfile

• Added DOCKER_REGISTRY build argument (defaults to docker.io)
• Updated base images to use the configurable registry:
  • golang:${GO_TAG} → ${DOCKER_REGISTRY}/library/golang:${GO_TAG}
  • alpine:latest → ${DOCKER_REGISTRY}/library/alpine:latest

Makefile

• Added CONTAINER variable (defaults to docker) to allow using alternative container tools like podman
• Updated all docker commands to use $(CONTAINER) variable
• Added --build-arg=DOCKER_REGISTRY=$(DOCKER_REGISTRY_SERVER) to all build commands
• Affected targets:
  • deploy-kind
  • docker-build
  • docker-push
  • docker-build-dev

Motivation

These changes enable:

1. Air-gapped/private registry support: Organizations using private Docker registries can now easily configure the base image source
2. Alternative container runtimes: Users can choose between Docker, Podman, or other compatible tools
3. Flexibility: Better support for different deployment environments and security requirements