Revert "Remove rhel.6"

This reverts commit 58537a3b9f9d2bf77e85a5db7f07da38c72ce14f.

eng/docker/rhel.6/Dockerfile

@@ -0,0 +1,34 @@
+#
+# 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:centos-6-376e1a3-20174311014331
+
+# yum doesn't work with the special curl version we have in the base docker image,
+# so we remove /usr/local/lib from the library path for this command
+RUN yum -q -y install sudo
+
+# Setup User to match Host User, and give superuser permissions
+ARG USER_ID=0
+RUN useradd -m code_executor -u ${USER_ID} -g 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 library path to make CURL and ICU libraries that are in /usr/local/lib visible
+ENV LD_LIBRARY_PATH /usr/local/lib
+
+# Set working directory
+ARG WORK_DIR
+WORKDIR ${WORK_DIR}
+
+# Set up Azure Artifacts credential provider
+RUN wget -qO- https://raw.githubusercontent.com/Microsoft/artifacts-credprovider/master/helpers/installcredprovider.sh | bash