Change Mount Detection to read /proc/mounts instead of using Device IDs

[caleblloyd]

Presently, fixuid prevents recursing into mounts by calling sys, ok := fileInfo.Sys().(*syscall.Stat_t) and reading the device ID from sys.Dev. If the Device ID of a current file or directory is different than the Device ID of the initially searched directory, the current file or directory is considered to be on a different mount and recursion into the mount is prevented.

I am now seeing a test failure on TravisCI, files are showing up with different Device IDs than their parent directories, even though the files and directories reside on the same mount:
https://travis-ci.org/boxboat/fixuid/builds/327907254

Logging output says that all of the directories in the / mount are on Device ID 41 but all of the files in the / mount are on Device ID 2049

I wondered if it was anything to do with the Storage Driver so I setup Overlay2 backed by EXT4 on my machine, and I could not recreate. At this point, I am not sure what environmental difference Travis CI has to cause this.

It seems that it would be more reliable to detect what mount a file is part of by parsing /proc/mounts instead of relying on the Device ID in the inode

[caleblloyd]

I have tried to recreate using the same storage driver and Docker Engine version as Travis CI and am unable to:

Travis CI: Tests fail

Server Version: 17.09.0-ce
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Kernel Version: 4.4.0-101-generic
Operating System: Ubuntu 14.04.5 LTS

My Laptop: All tests pass

Server Version: 17.09.0-ce
Storage Driver: overlay2
 Backing Filesystem: extfs
 Supports d_type: true
 Native Overlay Diff: true
Kernel Version: 4.13.0-21-generic
Operating System: Ubuntu 16.04.3 LTS

At this point I am wondering if it is a kernel difference in the implementation of Overlay2 or an option that TravisCI creates their EXT4 partitions with