fix: pass USE_SDL=1 to docker-build.sh and bundle libSDL3
The oven Docker build was missing USE_SDL=1 in the make invocation,
causing the binary to fail to compile with SDL3 headers. Also adds
libSDL3.so.0 to the initramfs lib bundling.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>