Fix: Remove buildx to fix --load flag error (fixes #149)

[wkoszek]

Summary

Remove use: buildx from Docker configuration to fix the --load flag error that occurs with multi-platform builds.

Problem

When using buildx with multi-platform builds, GoReleaser tries to use the --load flag which is incompatible:

docker build failed: failed to build ghcr.io/bsubio/cli:v0.15.0-arm64: 
exit status 125: unknown flag: --load

The --load flag only works for single-platform builds that are loaded into the local Docker daemon. Multi-platform builds must be pushed directly to a registry.

Solution

Remove use: buildx and let GoReleaser build each architecture separately using standard Docker build. Each architecture is built individually, then docker_manifests combines them into multi-arch manifests.

How It Works Now

1. Build linux/amd64 image separately with standard Docker
2. Build linux/arm64 image separately with standard Docker
3. Push both images to registry
4. Create docker_manifests to combine both into multi-arch images
5. Users pull multi-arch images that automatically select the right architecture

Result

• ✅ Multi-arch support maintained (amd64 + arm64)
• ✅ No --load flag conflicts
• ✅ Images pushed directly to registry
• ✅ Docker manifests work correctly

Technical Details

• Without use: buildx, GoReleaser uses standard docker build for each architecture
• The goarch field ensures the correct Go binary is used in each image
• docker_manifests section creates the multi-arch manifests after individual builds

Fixes #149

[claude]

Review

Correct fix. Removing use: buildx solves the --load flag conflict.

GoReleaser builds each arch separately with standard Docker, then combines via manifests (lines 280-299). This is the right approach for multi-arch.

One concern: Dockerfile at .goreleaser.yaml:246,264 uses ubuntu:22.04 base. Consider FROM scratch or alpine for smaller images, unless Ubuntu packages are required.

Ship it.