Provide ready-to-use images

[blueyed]

I think we should provide an ready-to-use image already, e.g. built from the example Dockerfile (https://github.com/tweekmonster/vim-testbed/blob/master/example/Dockerfile), and then some lighter images with only Vim or Neovim master.

That would allow for easy use, without you having to build your own image first.

Probably best after #1 and #8 are done.

[tweekmonster]

I did make this: https://hub.docker.com/r/tweekmonster/ubuntu-vims/

But yeah, a light weight image just for the latest Vim/Neovim builds would be cool.