Add the Android app shell; cap cross-build parallelism

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

docker/android/build.sh

@@ -46,6 +46,19 @@ ARGS=(
     -w /work
 )
 
+# Build parallelism. A full cross-compile will otherwise take every thread on
+# the host and make the machine unusable for the length of the build, which is
+# a poor trade when it is running in the background.
+#
+# Both halves are needed: CARGO_BUILD_JOBS caps how many rustc processes cargo
+# starts, while --cpus caps what the container gets no matter what any nested
+# build script decides to spawn (cc, cmake, and ring's asm build all parallelise
+# on their own account and do not consult cargo).
+JOBS="${DARKROOM_BUILD_JOBS:-8}"
+if [[ "${JOBS}" != "0" ]]; then
+    ARGS+=(--cpus "${JOBS}" -e "CARGO_BUILD_JOBS=${JOBS}")
+fi
+
 # Rootless podman already maps the host user; docker needs it stated.
 if [[ "${ENGINE}" == "docker" ]]; then
     ARGS+=(--user "$(id -u):$(id -g)")