Add unstable Dockerfile for CentOS 7

docker/launch.sh

@@ -9,7 +9,7 @@ fi
 
 # Build specified distributions
 if [[ -z $DISTROS ]]; then
-    DISTROS="ubuntu14.04 ubuntu16.04"
+    DISTROS="ubuntu14.04 ubuntu16.04 centos7"
 fi
 
 for build in $BUILDS; do

docker/unstable/centos7/Dockerfile

@@ -0,0 +1,6 @@
+FROM powershell/powershell:stable-centos7
+MAINTAINER Andrew Schwartzmeyer <andschwa@microsoft.com>
+
+COPY bootstrap.ps1 /
+RUN powershell -f bootstrap.ps1 \
+    && yum install -y PowerShell/powershell*.rpm