Cap both halves of the container build: CARGO_BUILD_JOBS limits how many
rustc processes cargo starts, while --cpus limits what the container gets
regardless of what nested build scripts spawn — cc, cmake, and ring's asm
build all parallelise on their own account and do not consult cargo. Without
both, a full cross-compile takes every thread on the host and makes the
machine unusable for the length of a background build.
Assisted-by: LLM