Docs: Fix incomplete instructions for building using docker

Harshal Patil · Harshal Patil · commit ef449aa38edc · 2018-06-13T13:38:57.000+05:30
Fixes: #2396 Signed-off-by: Harshal Patil <harshal.patil@in.ibm.com>
diff --git a/BUILDING.md b/BUILDING.md
@@ -106,7 +106,7 @@ You can build an image from this `Dockerfile`:
 FROM golang
 
 RUN apt-get update && \
-    apt-get install btrfs-tools
+    apt-get install -y btrfs-tools libseccomp-dev
 ```
 
 Let's suppose that you built an image called `containerd/build`. From the
