feat(docker): support native multi-arch (amd64/arm64) builds

[jtmcdole]

• Update Dockerfile.base to exclude prepackaged cache on arm64/aarch64 and trigger native SDK bootstrapping.
• Exclude .pub-preload-cache by default on all architectures to optimize size.
• Update Dockerfile to download yq dynamically based on target architecture.
• Refactor cloudbuild.yaml to use docker buildx with QEMU emulation and direct manifest-list pushing.
• Remove hardcoded amd64 platform constraints from docker-compose.yml.
• Update README.md to document arm64 capabilities and simplify local commands.

fixes: flutter/flutter#188873

[gemini-code-assist]

Code Review

This pull request introduces multi-architecture support (linux/amd64 and linux/arm64) for the Flutter Docker environment by updating the Dockerfiles, README, docker-compose, and transitioning the Cloud Build pipeline to use Docker Buildx with QEMU. Feedback on these changes highlights critical issues in cloudbuild.yaml where using a static builder name across separate build steps will cause container name conflicts and build failures. Additionally, improvements are suggested to avoid unquoted variables for word-splitting in Dockerfile.base and to pin the yq binary to a specific version in Dockerfile to ensure deterministic builds.

[jtmcdole]

/gemini review again.

[gemini-code-assist]

Code Review

This pull request adds multi-architecture support (linux/amd64 and linux/arm64) to the Flutter Docker development environment. It updates the Dockerfiles to dynamically handle architecture-specific downloads and cache exclusions, refactors the Cloud Build pipeline to build multi-arch images using docker buildx, and updates the documentation and docker-compose configuration. Feedback is provided to improve the robustness of local builds in Dockerfile.base by falling back to dpkg --print-architecture if TARGETARCH is not set.