Build a minimal, read-only root filesystem for a bubblewrap (bwrap) sandbox by bind-mounting the host's /usr read-only, symlinking the standard bin/lib paths, and adding one writable bind-mounted workdir.

Documented steps

1. Run the official minimal example verbatim: `bwrap --ro-bind /usr /usr --symlink usr/lib64 /lib64 --proc /proc --dev /dev --unshare-pid --new-session bash` — this reuses the host's /usr read-only. (source: README.md 'Usage' section, https://raw.githubusercontent.com/containers/bubblewrap/main/README.md)
2. Add the remaining merged-/usr symlinks for a fuller environment: `--symlink usr/lib /lib --symlink usr/bin /bin --symlink usr/sbin /sbin`.
3. Confirm the root is actually read-only: inside the sandbox, `touch /usr/testfile` should fail with a read-only filesystem error, since `--ro-bind` mounts SRC 'readonly on DEST' per bwrap.xml.
4. Add exactly one writable working directory: `--bind /home/user/workdir /work --chdir /work`; unlike `--ro-bind`, `--bind` leaves the mount writable (bwrap.xml).
5. If the host is not a merged-/usr distro, bind /bin, /lib, /lib64, /sbin directly instead of symlinking into /usr: `--ro-bind /bin /bin --ro-bind /lib /lib --ro-bind /lib64 /lib64 --ro-bind /sbin /sbin`.
6. For a chroot-style target rootfs rather than the live host tree, pre-create the lib64->usr/lib64 symlink inside the target rootfs itself instead of using `--symlink`, exactly as the README recommends for that case.