Enable building arbitrary forks and branches

docker/unstable/centos7/Dockerfile

@@ -1,6 +1,9 @@
 FROM powershell/powershell:stable-centos7
 MAINTAINER Andrew Schwartzmeyer <andschwa@microsoft.com>
 
+ARG fork=PowerShell
+ARG branch=master
+
 COPY bootstrap.ps1 /
 RUN powershell -f bootstrap.ps1 \
     && yum install -y PowerShell/powershell*.rpm