Description

Running make results in fatal error: btrfs/ioctl.h: No such file or directory
Steps to reproduce the issue:

1. Follow Docker "Build containerd" instructions

Describe the results you received:

root@368d77c382fa:/go/src/github.com/containerd/containerd# make
+ bin/ctr
+ bin/containerd
# github.com/containerd/containerd/vendor/github.com/containerd/btrfs
vendor/github.com/containerd/btrfs/btrfs.go:38:10: fatal error: btrfs/ioctl.h: No such file or directory
 #include <btrfs/ioctl.h>
          ^~~~~~~~~~~~~~~
compilation terminated.

Describe the results you expected:
A successful build

Output of containerd --version:

containerd github.com/containerd/containerd v1.3.0-86-gc80fa7df.m c80fa7df17ad8979c2e45a3550d561aa1151b85f.m

Proposed solution:
It appears that btrfs/ioctl.h is in libbtrfs-dev instead of btrfs-tools. Change the Dockerfile in BUILDING.md in the "Build containerd" section to include libbtrfs-dev instead of btrfs-tools.