test-in-docker: Support for multiple ZSH versions

-z --zsh can specify one of the versions of ZSH we can get from
   centos and ubuntu

docker/base-5.1.1/Dockerfile

@@ -0,0 +1,35 @@
+FROM ubuntu:16.04
+
+RUN \
+  apt-get update && \
+  echo 'golang-go golang-go/dashboard boolean false' | debconf-set-selections && \
+  DEBIAN_FRONTEND=noninteractive apt-get install -y \
+  curl \
+  git \
+  zsh \
+  mercurial \
+  subversion \
+  golang \
+  jq \
+  nodejs \
+  ruby \
+  python \
+  python-virtualenv \
+  sudo \
+  locales
+
+RUN adduser --shell /bin/zsh --gecos 'fred' --disabled-password fred
+RUN locale-gen "en_US.UTF-8"
+
+COPY docker/fred-sudoers /etc/sudoers.d/fred
+
+USER fred
+WORKDIR /home/fred
+ENV LANG=en_US.UTF-8
+ENV TERM=xterm-256color
+ENV DEFAULT_USER=fred
+ENV POWERLEVEL9K_ALWAYS_SHOW_CONTEXT=true
+
+RUN touch .zshrc
+
+CMD ["/bin/zsh", "-l"]