Mount volume at /pnpm-store and set PNPM_STORE_DIR and
COREPACK_HOME env vars in job containers so pnpm and corepack
both write to the cached volume. Corepack cache avoids
re-downloading pnpm binary each run.
Mount a named Docker volume (pnpm-store) into every job container
at the default pnpm store path. The volume persists in the DinD
sidecar across job runs, so pnpm install reuses cached packages.
