buildah: 3.0.0, 2.1.1: rootless Podman cant run "expect" (The system has no more ptys.) [regression]

Since few releases ago (going several weeks back) I can’t (re-)build a particular container image any more due to expect binary failing (in spawn command) as follows in rootless mode:

The system has no more ptys.  Ask your system administrator to create more.

(Formerly this container image was building fine by the older release of Podman).

expect(1) man page mentions the following:

Internally, spawn uses a pty, initialized the same way as the user’s tty.

# ls -l /dev/pts
total 0
crw--w---- 1 root tty  136, 0 Feb  7 14:18 0
crw-rw-rw- 1 root root   5, 2 Feb  7 14:18 ptmx

I’m not sure whether this have something to do with kernel, runc, crun or other system components. I’ve tried with runc and crun but reproduced the problem with both of them. Podman 2.0.6 on Debian “testing”/“unstable” had no such problem.

Here is the current output of podman info:

host:                                                                                                                                                                           
  arch: amd64                                                                                                                                                                   
  buildahVersion: 1.19.3                                                                                                                                                        
  cgroupManager: systemd                                                                                                                                                        
  cgroupVersion: v2                                                                                                                                                             
  conmon:                                                                                                                                                                       
    package: 'conmon: /usr/bin/conmon'                                                                                                                                          
    path: /usr/bin/conmon                                                                                                                                                       
    version: 'conmon version 2.0.25, commit: unknown'                                                                                                                           
  cpus: 4                                                                                                                                                                       
  distribution:                                                                                                                                                                 
    distribution: debian                                                                                                                                                        
    version: unknown                                                                                                                                                            
  eventLogger: journald                                                                                                                                                         
  hostname: deblab                                                                                                                                                              
  idMappings:                                                                                                                                                                   
    gidmap:                                                                                                                                                                     
    - container_id: 0                                                                                                                                                           
      host_id: 1000                                                                                                                                                             
      size: 1                                                                                                                                                                   
    - container_id: 1                                                                                                                                                           
      host_id: 427680                                                                                                                                                           
      size: 65536                                                                                                                                                               
    uidmap:                                                                                                                                                                     
    - container_id: 0                                                                                                                                                           
      host_id: 1000                                                                                                                                                             
      size: 1                                                                                                                                                                   
    - container_id: 1                                                                                                                                                           
      host_id: 427680                                                                                                                                                           
      size: 65536                                                                                                                                                               
  kernel: 5.10.0-2-amd64                                                                                                                                                        
  linkmode: dynamic                                                                                                                                                             
  memFree: 1301323776                                                                                                                                                           
  memTotal: 67363360768                                                                                                                                                         
  ociRuntime:                                                                                                                                                                   
    name: crun                                                                                                                                                                  
    package: 'crun: /usr/bin/crun'                                                                                                                                              
    path: /usr/bin/crun                                                                                                                                                         
    version: |-                                                                                                                                                                 
      crun version 0.17                                                                                                                                                         
      commit: 0e9229ae34caaebcb86f1fde18de3acaf18c6d9a                                                                                                                          
      spec: 1.0.0                                                                                                                                                               
      +SYSTEMD +SELINUX +APPARMOR +CAP +SECCOMP +EBPF +YAJL                                                                                                                     
  os: linux                                                                                                                                                                     
  remoteSocket:                                                                                                                                                                 
    exists: true                                                                                                                                                                
    path: /run/user/1000/podman/podman.sock                                                                                                                                     
  security:                                                                                                                                                                     
    apparmorEnabled: false
    capabilities: CAP_CHOWN,CAP_DAC_OVERRIDE,CAP_FOWNER,CAP_FSETID,CAP_KILL,CAP_NET_BIND_SERVICE,CAP_SETFCAP,CAP_SETGID,CAP_SETPCAP,CAP_SETUID,CAP_SYS_CHROOT
    rootless: true
    seccompEnabled: true
    selinuxEnabled: false
  slirp4netns:
    executable: /usr/bin/slirp4netns
    package: 'slirp4netns: /usr/bin/slirp4netns'
    version: |-
      slirp4netns version 1.0.1
      commit: 6a7b16babc95b6a3056b33fb45b74a6f62262dd4
      libslirp: 4.4.0
  swapFree: 15508434944
  swapTotal: 17495486464
  uptime: 14h 13m 26.36s (Approximately 0.58 days)
registries:
  search:
  - docker.io
  - registry.fedoraproject.org
store:
  configFile: ~/.config/containers/storage.conf
  containerStore:
    number: 0
    paused: 0
    running: 0
    stopped: 0
  graphDriverName: overlay
  graphOptions:
    overlay.mount_program:
      Executable: /usr/bin/fuse-overlayfs
      Package: 'fuse-overlayfs: /usr/bin/fuse-overlayfs'
      Version: |-
        fusermount3 version: 3.10.1
        fuse-overlayfs: version 1.4
        FUSE library version 3.10.1
        using FUSE kernel interface version 7.31
  graphRoot: ~/.local/share/containers/storage
  graphStatus:
    Backing Filesystem: btrfs
    Native Overlay Diff: "false"
    Supports d_type: "true"
    Using metacopy: "false"
  imageStore:
    number: 15
  runRoot: /run/user/1000
  volumePath: ~/.local/share/containers/storage/volumes
version:
  APIVersion: 3.0.0
  Built: 0
  BuiltTime: Thu Jan  1 10:00:00 1970
  GitCommit: ""
  GoVersion: go1.15.7
  OsArch: linux/amd64
  Version: 3.0.0-rc2

CC: @siretart.