fix: Use TARGETARCH for multi-platform builds

[lexfrei]

Summary

Enable ARM64 and other architecture support by using Docker's automatic TARGETARCH variable instead of hardcoded GOARCH=amd64.

Problem

The current Dockerfile hardcodes ARG GOARCH=amd64, which prevents building images for other architectures like arm64 using docker buildx build --platform linux/arm64.

Solution

• Use TARGETARCH which is automatically set by Docker buildx during multi-platform builds
• Fall back to amd64 for backward compatibility with non-buildx builds

Changes

# Before
ARG GOARCH=amd64

# After
ARG TARGETARCH
ARG GOARCH=${TARGETARCH:-amd64}

Testing

Build for multiple platforms:

docker buildx build --platform linux/amd64,linux/arm64 -t kilo:test .

[squat]

Kilo already supports multi-platform manifests, the CI just doesn't use docker buildx as the underlying tooling to accomplish the builds. Check out the available architectures for any tag on Docker Hub: https://hub.docker.com/layers/squat/kilo/latest/images/sha256-0d0987b6cc77f0d24c82fd13d34585c3db3a3a94efb01532945c59f3fae58c19

Or is the concrete goal of this PR to enable using buildx?