fix: respect Docker target architecture

[anschmieg]

Summary

Remove the amd64 default from TARGETARCH in the Dockerfile so Docker Buildx can pass the target architecture during multi-platform builds.

This is a minimal fix for the published linux/arm64 image currently containing an x86-64 binary, which causes native Linux ARM64 hosts to fail with exec format error.

Closes #6

Test plan

• go test ./... passes locally
• Built a test Coolify Dockerfile deployment from this fork on a Linux ARM64 VPS host; deployment completed and app started successfully
• App logs from the ARM64 host show:
  • config loaded
  • watching config file for changes
  • starting server, listen: 0.0.0.0:8080

Notes

The Coolify test branch only added a root Dockerfile copy because Coolify's API did not accept the nested Dockerfile path during testing. This PR intentionally keeps the upstream change to the single relevant Dockerfile line only.