Add subpath support to volumes in --mount option
          #1323
        
          
      
  Add this suggestion to a batch that can be applied as a single commit.
  This suggestion is invalid because no changes were made to the code.
  Suggestions cannot be applied while the pull request is closed.
  Suggestions cannot be applied while viewing a subset of changes.
  Only one suggestion per line can be applied in a batch.
  Add this suggestion to a batch that can be applied as a single commit.
  Applying suggestions on deleted lines is not supported.
  You must change the existing code in this line in order to create a valid suggestion.
  Outdated suggestions cannot be applied.
  This suggestion has been applied or marked resolved.
  Suggestions cannot be applied from pending reviews.
  Suggestions cannot be applied on multi-line comments.
  Suggestions cannot be applied while the pull request is queued to merge.
  Suggestion cannot be applied right now. Please check back later.
  
    
  
    
Add subpath support to volumes in
--mountoptionTo allow subpath mount as describes in Compose Specs,
refer_volume_over_mountis changed from True to FalseCompose entries exemple:
Contributor Checklist:
If this PR adds a new feature that improves compatibility with docker-compose, please add a link
to the exact part of compose spec that the PR touches.
For any user-visible change please add a release note to newsfragments directory, e.g.
newsfragments/my_feature.feature. See newsfragments/README.md for more details.
All changes require additional unit tests.