Updated Vim version in Dockerfile and made UpdateTagStack clearer

The `settagstack` and `gettagstack` functions don't exist prior to Vim
8.1.0519.  And the function definition was unclear whether it intended
  to grab the *old* or the *new* file/line/col.

Dockerfile

@@ -1,7 +1,7 @@
 FROM tweekmonster/vim-testbed:latest
 
 RUN install_vim -tag v8.0.0027 -build \
-                -tag v8.1.0204 -build \
+                -tag v8.1.0519 -build \
                 -tag neovim:v0.2.0 -build \
                 -tag neovim:v0.3.0 -build
 

autoload/ale/definition.vim

@@ -24,10 +24,12 @@ function! ale#definition#UpdateTagStack() abort
     let l:should_update_tagstack = exists('*gettagstack') && exists('*settagstack') && g:ale_update_tagstack
 
     if l:should_update_tagstack
-        let l:from = [bufnr('%'), line('.'), col('.'), 0]
+        " Grab the old location (to jump back to) and the word under the
+        " cursor (as a label for the tagstack)
+        let l:old_location = [bufnr('%'), line('.'), col('.'), 0]
         let l:tagname = expand('<cword>')
         let l:winid = win_getid()
-        call settagstack(l:winid, {'items': [{'from': l:from, 'tagname': l:tagname}]}, 'a')
+        call settagstack(l:winid, {'items': [{'from': l:old_location, 'tagname': l:tagname}]}, 'a')
         call settagstack(l:winid, {'curidx': len(gettagstack(l:winid)['items']) + 1})
     endif
 endfunction