Add ARM32 (armv7) Multi-Architecture Docker Support

Summary

This PR adds ARM32 (armv7) architecture support to Fabio's Docker images and makes the build process fully multi-architecture aware. Docker images will now be built for three architectures: amd64, arm64, and armv7.

Changes Made

1. Updated Dockerfile

• Made consul/vault binary downloads architecture-aware using ${TARGETARCH} variable
• Downloads correct binaries for each target platform (amd64, arm64, arm)
• Maintains original Dockerfile structure and test execution

2. Updated .goreleaser.yml

• Added ARM32 (armv7) Docker build configuration
• Creates multi-architecture manifests combining all three platforms:
  • fabiolb/fabio:latest → amd64, arm64v8, armv7
  • fabiolb/fabio:{{ .Version }} → amd64, arm64v8, armv7

3. Enhanced GitHub Actions Workflows

• Added Docker Buildx setup to build workflow
• Workflows now support building and pushing multi-arch images on release

Why This Matters

1. Broader Hardware Compatibility

• Enables Fabio to run on ARM32 devices like Raspberry Pi 2/3/Zero 2
• Supports older ARM-based servers and embedded systems
• Expands deployment options for IoT and edge computing scenarios

2. Cost-Effective Edge Deployments

• ARM32 devices are significantly cheaper than ARM64 or x86_64 alternatives
• Perfect for home labs, small-scale deployments, and educational purposes
• Reduces hardware costs for development and testing environments

3. Legacy Hardware Support

• Extends the life of existing ARM32 infrastructure
• No need to upgrade hardware to run modern Fabio versions
• Supports organizations with investment in ARM32-based systems

4. Seamless User Experience

• Docker automatically pulls the correct architecture image
• No manual intervention required - just docker pull fabiolb/fabio:latest
• Single manifest works across all platforms

5. Complete Platform Coverage

• Covers the full spectrum of modern ARM devices (32-bit and 64-bit)
• Matches the breadth of platforms already supported in binary releases
• Ensures Docker deployment parity with standalone binary deployment options

Testing

The Docker build process has been tested for all three architectures:

docker buildx build --platform linux/amd64,linux/arm64,linux/arm/v7 \
  -t fabiolb/fabio:latest --push .

All architectures build successfully with full test execution.

Breaking Changes

None. This is a purely additive change that expands platform support.