build: support native architecture builds on ARM hosts

[sheurich]

Removes forced amd64 cross-compilation. Builds now run natively for the host architecture.

This is a focused subset of #8389, extracting only the changes needed for native ARM builds without CI workflow modifications.

The upstream CI currently runs on amd64 only. A future PR can add parallel arm64 runners to produce multi-platform binaries and container images.

Changes

• tools/container-build.sh: Detect host architecture instead of forcing amd64 on ARM.
• Containerfile: Add TARGETPLATFORM arg for correct Go binary download.
• tools/make-deb.sh: Accept ARCH from caller for deb packaging. ARCH env var is now required.
• Artifact naming: Standardized to amd64/arm64 (previously x86_64).
• Bug fix: Removed errant trailing . from tar command.
• Cleanup: Added .github to .dockerignore and build artifact patterns to .gitignore.

Testing

Native builds work on both architectures:

GO_VERSION=1.25.5 ./tools/container-build.sh
# arm64 host → arm64 artifacts ✓
# amd64 host → amd64 artifacts ✓

Cross-compilation via QEMU is not supported (Go assembler segfaults under emulation). For multi-platform binaries, use native runners for each architecture.

[aarongable]

Apologies for taking so long to get back to this. LGTM, and we're happy to merge this. Note, however, that we've been working on container-based architecture and don't plan to invest further in our debian-based deploy system.