Sessions start with a default root filesystem

Every Bash session now starts with a synthetic root filesystem: a read-only /etc (os-release, passwd, group, hostname, hosts, shells, timezone), static /proc files (cpuinfo, meminfo, version, loadavg), /root and /dev/zero. /bin and /usr/bin list the builtins, so a /usr/bin/env shebang runs the builtin and type, which and command -v report /usr/bin/NAME.

Session files take precedence, and the layer uses no quota and is never snapshotted. Turn it off with:

let bash = Bash::builder().rootfs(false).build();

See Filesystem and #2525.