Description

BUILDING.md file has incomplete instructions on how to build using docker

Steps to reproduce the issue:

1. NA

Describe the results you received:
Incorrect instructions leads to build process breaking

Describe the results you expected:
In order to build using docker, the image needs to have btrfs-tools and libseccomp-dev. Also, it should be apt-get install -y

Output of containerd --version:
NA