See https://bugs.launchpad.net/oslo.rootwrap/+bug/1760471, in particular
comment #1 for an explanation of why inheriting the defaults of the
docker daemon can cause poor performance:
The performance difference likely comes from close_fds=True of subprocess.
Popen. On Python 2, Popen calls close(fd) on all file descriptors from 3 to
SC_OPEN_MAX. On my Fedora 27 "host", SC_OPEN_MAX is 1,024. But in docker,
SC_OPEN_MAX is... 1,048,576: 1,000x larger. On Python 3, Popen is smarter. On
Linux, it lists the content of /proc/self/fd/ to only close open file
descriptors. It doesn't depend on SC_OPEN_MAX value.
Change-Id: Iefef6039644192420abbd3bf614329cbc0d9a62a
Closes-Bug: #1848737
Related-Bug: #1760471
Related-Bug: #1757556
Related-Bug: #1824020
ae5acc7f3c