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/Dockerfile

@@ -1,33 +0,0 @@
-FROM ubuntu:trusty
-
-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 \
-  node \
-  ruby \
-  python \
-  python-virtualenv
-
-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"]