Updated base image and added docker to readme

Author: Jake Ginnivan

README.md

@@ -17,6 +17,7 @@ works out the [semantic version][semver] of the commit being built.
 | **GitVersion.CommandLine** |     [![NuGet][gvc-badge]][gvc]            |       [![NuGet][gvc-pre-badge]][gvc]      |
 |                    **Gem** |       [![Gem][gem-badge]][gem]            |                      -                    |
 |               **Homebrew** |   [![homebrew][brew-badge]][brew]         |                      -                    |
+|               **Docker**   |   [gittools/gitversion][dockerhub]        |                      -                    |
 
 ## Compatibility
 GitVersion works on Mac, Linux with Mono and Windows.
@@ -84,3 +85,4 @@ from The Noun Project
 [faq]:             http://gitversion.readthedocs.org/en/latest/faq/
 [who]:             http://gitversion.readthedocs.org/en/latest/who/
 [gv-in-action]:    https://raw.github.com/GitTools/GitVersion/master/docs/img/README.png
+[dockerhub]:       https://hub.docker.com/r/gittools/gitversion/

src/DockerBase/Dockerfile

@@ -1,11 +1,13 @@
-FROM ubuntu:16.04
+FROM ubuntu
 
 MAINTAINER GitTools Maintainers <[email protected]>
 
 # Wheezy doesn't support glibc 2.15 which libgit2sharp requires
 # So we are going to install mono on ubuntu instead
 RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
+RUN apt-get update
+RUN apt-get install libcurl3 tzdata unzip curl git-all -y
 RUN echo "deb http://download.mono-project.com/repo/debian wheezy main" | tee /etc/apt/sources.list.d/mono-xamarin.list
 RUN apt-get update
-RUN apt-get install mono-complete libcurl3 tzdata unzip curl git-all -y
+RUN apt-get install mono-complete -y
 RUN cp /usr/share/zoneinfo/GMT /etc/localtime