Add a dockerfile for quick, manual testing.

Dockerfile.repro

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+FROM ultisnips
+
+RUN curl -fLo ~/.vim/autoload/plug.vim --create-dirs \
+    https://raw.githubusercontent.com/junegunn/vim-plug/master/plug.vim
+
+ADD doc/minimal_vimrc.vim /root/.vimrc
+
+RUN vim -c 'PlugInstall | qa'

doc/minimal_vimrc.vim

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+call plug#begin('~/.vim/plugged')
+
+Plug 'SirVer/ultisnips'
+Plug 'honza/vim-snippets'
+
+let g:UltiSnipsExpandTrigger="<tab>"
+let g:UltiSnipsJumpForwardTrigger="<c-b>"
+let g:UltiSnipsJumpBackwardTrigger="<c-z>"
+let g:UltiSnipsEditSplit="vertical"
+
+call plug#end()