Enable Alpine 3.6 CLI build

This change enables CLI build for Alpine 3.6 and also adds CI job for
it. It is mostly based on changes that were necessary to
enable building of rhel.6 CLI

scripts/docker/alpine.3.6/Dockerfile

@@ -0,0 +1,26 @@
+#
+# Copyright (c) .NET Foundation and contributors. All rights reserved.
+# Licensed under the MIT license. See LICENSE file in the project root for full license information.
+#
+
+# Dockerfile that creates a container suitable to build dotnet-cli
+FROM microsoft/dotnet-buildtools-prereqs:alpine-3.6-3148f11-20171119021156
+
+RUN apk add --no-cache sudo ncurses
+
+# Setup User to match Host User, and give superuser permissions
+ARG USER_ID=0
+RUN adduser -D -u ${USER_ID} code_executor root
+RUN echo 'code_executor ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
+
+# With the User Change, we need to change permssions on these directories
+RUN chmod -R a+rwx /usr/local
+RUN chmod -R a+rwx /home
+RUN chmod -R 755 /usr/bin/sudo
+
+# Set user to the one we just created
+USER ${USER_ID}
+
+# Set working directory
+WORKDIR /opt/code
+