Add Dockerfile for RHEL 6 builds

This change enables building CLI for RHEL 6 in docker container.

scripts/docker/rhel.6/Dockerfile

@@ -0,0 +1,42 @@
+#
+# 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-6-783abde-20171304101322
+
+# Install prerequisites for the git build below
+RUN yum -q -y install sudo expat-devel perl-devel autoconf gcc gcc-c++ gettext-devel
+
+# Compile and install a version of the git that supports the features that cli build needs
+RUN \
+    wget https://www.kernel.org/pub/software/scm/git/git-2.9.5.tar.gz && \
+    tar -xf git-2.9.5.tar.gz && \
+    cd  git-2.9.5 && \
+    make configure && \
+    ./configure --prefix=/usr/local --without-tcltk && \
+    make -j $(nproc --all) all && \
+    make install && \
+    cd .. && \
+    rm -r git-2.9.5
+
+# 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
+WORKDIR /opt/code
+