Fix ARM64 emulation support in all entrypoints

- Add shared setup-exec.sh script to handle ARM64 emulation logic
- Update scenario.sh and scenario2map.sh to use box64 emulation on ARM64
- Refactor docker-entrypoint.sh and docker-entrypoint-rootless.sh to use shared script
- Ensures all factorio binary calls work correctly on ARM64 platforms

Fixes #585

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker/files/setup-exec.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+# Setup EXEC variable for running Factorio with ARM64 emulation support
+# This script handles ARM64 emulation and can be combined with user switching as needed
+
+# If EXEC is not already set, initialize it
+if [[ -z "${EXEC:-}" ]]; then
+  EXEC=""
+fi
+
+if [[ -f /bin/box64 ]]; then
+  # Use an emulator to run on ARM hosts
+  # this only gets installed when the target docker platform is linux/arm64
+  EXEC="$EXEC /bin/box64"
+fi
+
+export EXEC