Minimal vimrc

With Vim/Neovim

First, create ~/vimrc.min with the following content

" Disable Vim's native pack feature
set packpath=

" Clone https://github.com/lambdalisue/fern.vim in somewhere
" and specify that directory to the below
set runtimepath^=~/ghq/github.com/lambdalisue/fern.vim

filetype plugin indent on
syntax on
"----------------------------------------------------------------

" Add extra settings here to reproduce the issue...

"----------------------------------------------------------------
echomsg "Custom minimal vimrc has loaded"

Then launch Vim/Neovim with

vim -N -u ~/vimrc.min

nvim -u ~/vimrc.min

Then add extra settings to reproduce the issue.

With Docker (Vim)

Create vimrc file like

if has('nvim')
  set runtimepath^=~/.vim
  set runtimepath+=~/.vim/after
  set packpath^=~/.vim
endif

packloadall

filetype plugin indent on
syntax on
"----------------------------------------------------------------

" Add extra settings here to reproduce the issue...

"----------------------------------------------------------------
echomsg "Custom minimal vimrc has loaded"

Then create Dockerfile like

FROM thinca/vim:v8.1.0451

ENV VIMPATH=/root/.vim \
    PACKPATH=/root/.vim/pack/test/start

RUN apk --no-cache add git
RUN git clone https://github.com/lambdalisue/fern.vim $PACKPATH/fern.vim

COPY vimrc /root/.vim/

Then build and run the Dockerfile like

docker build . -t fern-test
docker run --rm -it fern-test