Issue where sometimes the container doesn't exit

gistfile1.txt

$ docker run --volume "${PWD}:/home/tester/core" debianimage ps aux
Starting Xvfb...
Started Xvfb with PID 8
Running command: ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  3.0  0.0  21672  3256 ?        Ss   08:57   0:00 /bin/bash /home/tester/run-wrapper ps aux
root         8  0.0  0.0  19424   876 ?        R    08:57   0:00 Xvfb :1 -screen 0 1024x768x24
root         9  0.0  0.0  19104  2560 ?        R    08:57   0:00 ps aux
Finished
Stopping Xvfb...
Waiting for Xvfb to stop...
$ docker run --volume "${PWD}:/home/tester/core" debianimage ps aux
Starting Xvfb...
Started Xvfb with PID 8
Running command: ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  21672  3240 ?        Ss   08:57   0:00 /bin/bash /home/tester/run-wrapper ps aux
root         8  0.0  0.0  83432  2388 ?        R    08:57   0:00 Xvfb :1 -screen 0 1024x768x24
root         9  0.0  0.0  19104  2440 ?        R    08:57   0:00 ps aux
Finished
Stopping Xvfb...
Waiting for Xvfb to stop...
$ docker run --volume "${PWD}:/home/tester/core" debianimage ps aux
Starting Xvfb...
Started Xvfb with PID 7
Running command: ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  21672  3132 ?        Ss   08:57   0:00 /bin/bash /home/tester/run-wrapper ps aux
root         7  0.0  0.0  47876  1696 ?        R    08:57   0:00 Xvfb :1 -screen 0 1024x768x24
root         8  0.0  0.0  19104  2428 ?        R    08:57   0:00 ps aux
Finished
Stopping Xvfb...
Waiting for Xvfb to stop...
$ docker run --volume "${PWD}:/home/tester/core" debianimage ps aux
Starting Xvfb...
Started Xvfb with PID 8
Running command: ps aux
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  21672  3164 ?        Ss   08:57   0:00 /bin/bash /home/tester/run-wrapper ps aux
root         8  0.0  0.0  21668   300 ?        R    08:57   0:00 /bin/bash /home/tester/run-wrapper ps aux
root         9  0.0  0.0  19104  2492 ?        R    08:57   0:00 ps aux
Finished
Stopping Xvfb...
Waiting for Xvfb to stop...
(HANGS FOREVER)