e2e: Add missing headers for build

Signed-off-by: Christopher Crone <christopher.crone@docker.com>

Dockerfile.e2e

@@ -3,6 +3,7 @@ FROM golang:1.8.5-alpine3.6 as builder
 
 RUN apk add --update \
     bash \
+    btrfs-progs-dev \
     build-base \
     curl \
     lvm2-dev \