Add ALE to the Vim runtimepath in the Docker image so it can run tests.

test/vimrc

@@ -1,7 +1,10 @@
+" vint: -ProhibitSetNoCompatible
+
 " Load builtin plugins
 " We need this because run_vim.sh sets -i NONE
-set rtp=/home/vim,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,/home/vim/after
-set rtp+=/vader
+set runtimepath=/home/vim,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,/home/vim/after
+set runtimepath+=/home/ale
+set runtimepath+=/vader
 
 " The following is just an example
 filetype plugin indent on
@@ -18,4 +21,4 @@ set foldlevelstart=10
 set foldnestmax=10
 set ttimeoutlen=0
 
-let mapleader=','
+let g:mapleader=','