Optimize multi-arch docker image build

[maxsxu]

Motivation

Multi-arch builds (specifically arm64+amd64) using Docker Buildx are extremely slow when QEMU emulation is used for Go module download and compilation.
This PR optimizes the build by instructing Docker to use the native architecture of the build machine for builder stages (--platform=$BUILDPLATFORM), while using Go's built-in cross-compilation via TARGETOS and TARGETARCH for the produced binaries.

Modifications

• Added --platform=$BUILDPLATFORM to builder stages in Dockerfile, redhat.Dockerfile, operator.Dockerfile, and the sample Go function Dockerfile.
• Updated Go build commands to use TARGETOS and TARGETARCH inside Docker builds.
• Updated Makefile and images/**/build.sh to accept a PLATFORMS list.
• Kept single-platform builds on normal docker build, and used docker buildx build --push for multi-platform builds so manifest-list images can be published directly.
• Updated pulsarctl runner images to download the correct pulsarctl artifact for amd64 and arm64.
• Updated the release workflow to set up Buildx and publish linux/amd64 and linux/arm64 images.
• Restored the legacy local runner-image tags for single-platform CI builds so existing Trivy scans still work.

Verifying this change

• bash -n images/build.sh
• bash -n images/samples/build.sh
• make -n docker-build PLATFORMS=linux/amd64,linux/arm64
• make -n operator-docker-image PLATFORMS=linux/arm64
• docker build --platform linux/arm64 -f operator.Dockerfile -t function-mesh-verify:arm64 .
• docker buildx build --platform linux/amd64,linux/arm64 --output=type=cacheonly -f operator.Dockerfile .
• make docker-build-skip-test PLATFORMS=linux/arm64 IMG=function-mesh-verify:make

Documentation

• Updated repository docs for the new multi-platform build inputs and release behavior.

[Copilot]

Pull request overview

This PR speeds up multi-architecture (amd64+arm64) Docker builds by ensuring Go compilation happens on the native builder architecture (BUILDPLATFORM) while cross-compiling the resulting binaries for the target architecture (TARGETOS/TARGETARCH), and by updating build scripts/targets to support multi-platform Buildx builds.

Changes:

• Update Dockerfiles to use FROM --platform=$BUILDPLATFORM for builder stages and cross-compile with GOOS=$TARGETOS / GOARCH=$TARGETARCH.
• Update Makefile and images/**/build.sh scripts to accept a PLATFORMS list and use docker buildx build for multi-platform builds.
• Adjust pulsarctl runner images to download the correct pulsarctl artifact for amd64 vs arm64, and update release workflow to set up Buildx and build multi-platform images.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Dockerfile	Use native builder platform + Go cross-compilation for faster multi-arch builds.
redhat.Dockerfile	Same optimization for Red Hat certification image build.
operator.Dockerfile	Build operator binary inside Docker build (builder stage) and copy into runtime image.
Makefile	Add PLATFORMS support and switch to Buildx + push for multi-platform builds.
README.md	Document PLATFORMS usage for multi-platform image builds.
images/build.sh	Add PLATFORMS + multi-platform Buildx flow for runner images; enforce push requirements.
images/samples/build.sh	Add PLATFORMS + multi-platform Buildx flow for sample images; enforce push requirements.
images/samples/go-function-samples/Dockerfile	Use native builder platform + Go cross-compilation for sample image.
images/pulsar-functions-base-runner/pulsarctl.Dockerfile	Download arch-specific pulsarctl based on TARGETARCH.
images/pulsar-functions-python-runner/pulsarctl.Dockerfile	Download arch-specific pulsarctl based on TARGETARCH.
images/pulsar-functions-java-runner/pulsarctl.Dockerfile	Parameterize base image via BASE_IMAGE build arg.
images/pulsar-functions-go-runner/pulsarctl.Dockerfile	Parameterize base image via BASE_IMAGE build arg.
.github/workflows/release.yml	Set up Buildx and build/publish multi-platform images in release workflow.
docs/release_proocess.md	Note that release workflow publishes amd64+arm64 images.

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