Add Docker multi-architecture build documentation and examples

[Copilot]

References a workflow failure where hard-coded AMD64 package URLs caused ARM64 Docker builds to fail with package architecture (amd64) does not match system (arm64).

Solution Pattern

Use Docker's TARGETARCH build argument for dynamic architecture detection:

ARG TARGETARCH

RUN case "${TARGETARCH}" in \
      amd64) ARCH_SUFFIX="64bit" ;; \
      arm64) ARCH_SUFFIX="arm64" ;; \
      *) echo "Unsupported: ${TARGETARCH}" && exit 1 ;; \
    esac && \
    curl "https://example.com/plugin/ubuntu_${ARCH_SUFFIX}/package.deb" \
      -o "/tmp/package.deb" && \
    dpkg -i "/tmp/package.deb"

Changes

• docs/docker-multi-architecture-guide.md - Complete guide covering problem analysis, solution patterns, Docker build arguments, and best practices
• docs/docker-examples-README.md - Quick reference with practical examples for local and CI/CD builds
• Dockerfile.example - Working multi-stage Dockerfile demonstrating architecture-aware package installation for .NET applications
• .github/workflows/docker-build-example.yml - GitHub Actions workflow template with QEMU/Buildx setup for multi-platform builds
• SOLUTION-SUMMARY.md - Executive summary mapping the solution back to the original workflow failure

Key Principles

• Never hard-code architecture in package URLs
• Always use TARGETARCH for conditional logic
• Fail explicitly on unsupported architectures
• Use BuildKit and Buildx for multi-platform support

Original prompt

Reference: https://github.com/xpipe-io/xpipe-webtop/actions/runs/20578223451/job/59099982309

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