Make zsh start by default and allow overriding from script

Dockerfile

@@ -27,7 +27,10 @@ RUN cd /root
     sh -c "$(curl -fsSL https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh)" &&\
     ln -sf /root/zshrc/rc /root/.zshrc                                                          &&\
     git clone https://github.com/zsh-users/zsh-syntax-highlighting.git \
-    ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/plugins/zsh-syntax-highlighting
+    ${ZSH_CUSTOM:-~/.oh-my-zsh/custom}/plugins/zsh-syntax-highlighting                          &&\
+    chsh -s /bin/zsh
 
 RUN git clone --depth 1 https://github.com/junegunn/fzf.git /root/.fzf &&\
     /root/.fzf/install
+
+CMD ["/bin/zsh"]