Add docker container.

Because clang is outdated on many of my other machines.

Dockerfile

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+# C++ build environment. Useful with the following script that should
+# be run from a repo:
+##!/bin/sh
+#
+#CONTAINER="{CONTAINER:-cc-build:latest}"
+#
+#docker run -i -t -v $(pwd):/workspace ${CONTAINER} bash
+#
+# NB: Ubuntu 17.10 seems to be the only common distro with C++17 support.
+FROM ubuntu:17.10
+MAINTAINER kyle
+
+# install tools
+RUN apt-get -y update && apt-get -y install clang-5.0 autoconf automake libtool cmake ninja-build texinfo curl mg
+
+# make the env pick up the correct compilers.
+ENV CXX=clang++-5.0
+ENV CC=clang-5.0
+
+# set up workspace
+RUN mkdir /workspace
+WORKDIR /workspace
+
+# start compiling already
+CMD "bash"

cxe

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+#!/bin/sh
+
+CONTAINER="{CONTAINER:-cc-build:latest}"
+
+docker run -i -t -v $(pwd):/workspace ${CONTAINER} bash