docker mount /proc and /sys, use in_target_nofail

A couple of suites fail to debootstrap on gitlab.com (e.g. sid, stretch, and xenial). I'm using GitLab's CI to build systems with debootstrap.

I dug down the issue to the lines that handle mount -t proc none /proc in Docker environments. It's unclear to me why the behavior changes from suite to suite, but it might have something to do with dash. Replacing the in_target calls with in_target_nofail fixes the issue so it indicates the problem might just be in the parsing and handling logic of in_target. In any case, the current (if it worked) behavior is to report an error and ignore it -- which is incorrect IMO. The patch effectively handles the error silently as a bonus.

I've been running this patch on 12 suites (all those currently supported by Debian and Ubuntu) both locally and on gitlab.com for the last couple of days and everything builds correctly.