feat(docker): add simple config for trying or testing

[tunakasif]

I'm proposing a simple Docker blueprint for trying and testing the kickstart.nvim configuration. I believe it would be nice for people who wants to try the kickstart configuration, but do not want to mess with their machines or with their existing Neovim configurations. With the proposed container approach, it would be possible to enter a working kickstart.nvim demo by running the following:

git clone https://github.com/nvim-lua/kickstart.nvim.git \
    && cd kickstart.nvim \
    && docker build -t kickstart . \
    && docker run -it --rm kickstart