intermittent pexpect failures

[tfoote]

I ran rocker inside byobu and got the following traceback.

Immediately rerunning it did not cause the same error.

It appeared to be hung but this may be it downloading a bigger image in the background because it was a fresh machine. And now with the silent pull in the background pexpect might timeout due to no output from the process. If confirmed this will be related to #86

Successfully built e02c380413c7
running,  docker run -it --rm e02c380413c7
Traceback (most recent call last):
  File "/home/ubuntu/venv/bin/rocker", line 11, in <module>
    load_entry_point('rocker==0.2.3', 'console_scripts', 'rocker')()
  File "/home/ubuntu/venv/lib/python3.8/site-packages/rocker-0.2.3-py3.8.egg/rocker/cli.py", line 72, in main
  File "/home/ubuntu/venv/lib/python3.8/site-packages/rocker-0.2.3-py3.8.egg/rocker/core.py", line 249, in run
  File "/home/ubuntu/novnc-rocker/novnc_rocker/turbovnc.py", line 19, in precondition_environment
    detected_os = detect_os(cli_args['base_image'], print, nocache=cli_args.get('nocache', False))
  File "/home/ubuntu/venv/lib/python3.8/site-packages/rocker-0.2.3-py3.8.egg/rocker/os_detector.py", line 65, in detect_os
  File "/home/ubuntu/venv/lib/python3.8/site-packages/pexpect-4.8.0-py3.8.egg/pexpect/spawnbase.py", line 444, in read
    self.expect(self.delimiter)
  File "/home/ubuntu/venv/lib/python3.8/site-packages/pexpect-4.8.0-py3.8.egg/pexpect/spawnbase.py", line 343, in expect
    return self.expect_list(compiled_pattern_list,
  File "/home/ubuntu/venv/lib/python3.8/site-packages/pexpect-4.8.0-py3.8.egg/pexpect/spawnbase.py", line 372, in expect_list
    return exp.expect_loop(timeout)
  File "/home/ubuntu/venv/lib/python3.8/site-packages/pexpect-4.8.0-py3.8.egg/pexpect/expect.py", line 181, in expect_loop
    return self.timeout(e)
  File "/home/ubuntu/venv/lib/python3.8/site-packages/pexpect-4.8.0-py3.8.egg/pexpect/expect.py", line 144, in timeout
    raise exc
pexpect.exceptions.TIMEOUT: Timeout exceeded.
<pexpect.pty_spawn.spawn object at 0x7fd297ef8b20>
command: /usr/bin/docker
args: ['/usr/bin/docker', 'run', '-it', '--rm', 'e02c380413c7']
buffer (last 100 chars): b''
before (last 100 chars): ''
after: <class 'pexpect.exceptions.TIMEOUT'>
match: None
match_index: None
exitstatus: None
flag_eof: False
pid: 46996
child_fd: 5
closed: False
timeout: 30
delimiter: <class 'pexpect.exceptions.EOF'>
logfile: None
logfile_read: None
logfile_send: None
maxread: 2000
ignorecase: False
searchwindowsize: None
delaybeforesend: 0.05
delayafterclose: 0.1
delayafterterminate: 0.1
searcher: searcher_re:
    0: EOF