Optimize multi-arch docker image build

[maxsxu]

Motivation

Multi-arch builds (specifically arm64+amd64) using Docker Buildx are extremely slow when QEMU emulation is utilized for compiling code (like Go module downloading and compiling).
This PR optimizes the build by instructing Docker to use the native architecture of the build machine for the builder stage (--platform=$BUILDPLATFORM), while using Go's built-in cross-compilation features via TARGETOS and TARGETARCH. This significantly accelerates the build process.

Modifications

• Added --platform=$BUILDPLATFORM to builder stages in Dockerfile and redhat.Dockerfile.
• Overridden GOOS and GOARCH with $TARGETOS and $TARGETARCH in go build command.
• Appended --platform=$TARGETPLATFORM to the base images on final stages to ensure the runtime container matches the target platform.
• Fixed operator.Dockerfile to accept the platform parameter and select the right compiled artifact using TARGETARCH.
• Updated the Makefile build targets to use docker buildx build for native multi-arch building (--platform linux/amd64,linux/arm64) using --load to mimic the previous docker build functionality where the image gets loaded to the local daemon (useful for the next targets or CI testing).

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Documentation

Need to update docs?

• no-need-doc

  (No new features, just CI/Build system optimizations)