Added a Dockerfile for experimenting with precompiled bins

I put this together for my own purposes but sharing here as it's likely
useful for others. A couple of related thoughts:

* I'll update the docs with instructions for mounting providers for
  testing once puppetlabs-toy-chest#34 is
  resolved.
* I could have used the new multi-state builds to avoid some of the
  apt wranging, but left this way for now until multi-stage builds works
  it's way into a stable Docker release
* This only supports building from the precompiled binary. I'll take a
  run at converting the HACKING instructions at some point so you can
  build using Docker, that should be a separate PR

I also haven't pushed an image to Docker Hub. I would be happy to do
that if useful or hold off for now.

.dockerignore

@@ -0,0 +1 @@
+*

Dockerfile

@@ -0,0 +1,19 @@
+FROM ubuntu:latest
+MAINTAINER Gareth Rushgrove "[email protected]"
+
+ARG LIBRAL_DOWNLOAD_LOCATION="http://download.augeas.net/libral/"
+ARG LIBRAL_FILENAME="ralsh-latest.tgz"
+
+RUN apt-get update && \
+    apt-get install --no-install-recommends -y wget && \
+    wget "${LIBRAL_DOWNLOAD_LOCATION}${LIBRAL_FILENAME}" && \
+    tar zxvf "$LIBRAL_FILENAME" -C /opt && \
+    apt-get remove --purge -y wget && \
+    apt-get autoremove -y && \
+    apt-get clean && \
+    rm -rf /var/lib/apt/lists/* && \
+    rm "$LIBRAL_FILENAME"
+
+ENV PATH=/opt/ral/bin:$PATH
+
+ENTRYPOINT ["/opt/ral/bin/ralsh"]

README.md

@@ -45,6 +45,27 @@ In case you do need to build from source, which is not required for
 provider development, only if you want to work on the core libral library,
 [this document](HACKING.md) contains instructions on building `libral`.
 
+
+### Docker
+
+You can also try out `libral` in the context of a Docker container. The
+Dockerfile in the repository allows for building an image quickly based
+on the above mentioned precompiled tarball.
+
+```
+docker build -t puppet/libral .
+```
+
+Running this can then be done with Docker, for instance the following
+invocation will launch `ralsh` in the context of the container.
+
+```
+docker run --rm -t puppet/libral
+```
+
+This is intended for exploring the CLI and experimenting with providers.
+
+
 ## Usage
 
 After you build `libral`, or after you download and unpack the