Add docker binary to the docker container that is going to build the docker image

Author: tbouron

Dockerfile

@@ -18,7 +18,7 @@
 FROM maven:3.5.2-jdk-8-alpine
 
 # Install necessary binaries to build brooklyn-dist
-RUN apk add --no-cache git rpm dpkg
+RUN apk add --no-cache git rpm dpkg docker
 
 # Make sure the /var/tmp (for RPM build) is writable for all users
 RUN mkdir -p /var/tmp/ && chmod -R 777 /var/tmp/