Add remote Docker build mode

[FrontMage]

Summary

• add BUILD_IN_CONTAINER support to delegate builds to a remote Docker host
• add a Ubuntu 24.04 builder image with aarch64 GNU cross tools and build dependencies
• keep remote downloads/work/output/rootfs directories persistent under Documents
• support proxy auto/always/off modes and Dockerfile hash-based image reuse

Testing

• bash -n main/main.sh lib/container.sh start-build.sh
• BUILD_IN_CONTAINER=1 CONTAINER_SSH_PASSWORD=... ./start-build.sh zlib
• BUILD_IN_CONTAINER=1 CONTAINER_SSH_PASSWORD=... ./start-build.sh -r zlib
• BUILD_IN_CONTAINER=1 CONTAINER_SSH_PASSWORD=... CONTAINER_PROXY_MODE=always ./start-build.sh zlib
• verified remote rootfs contains zlib headers, pkg-config file, and libz.so.1.3.1