It's easy really, use toolbox to install then copy the files into the host:

# toolbox yum install bash-completion \
            && toolbox wget https://raw.githubusercontent.com/docker/docker/master/contrib/completion/bash/docker -O /usr/share/bash-completion/completions/docker \
            && toolbox cp /usr/share/bash-completion /media/root/var/ -R  \
            && source /var/bash-completion/bash_completion 
            

Then, assuming your .bashrc is still symlinked to ../../usr/share/skel/.bashrc, dereference it and copy it over the link:

# cp $(readlink .bashrc) .bashrc.new && mv .bashrc.new .bashrc
            

Then add bash completion to the resulting file:

# echo "source /var/bash-completion/bash_completion" >> ~/.bashrc
            

Now BASH completion will be loaded into your shell when it starts.