Can we mount sub-directories of a named volume in docker?

Question

The docker-compose file https://docs.docker.com/compose/compose-file/#/volumes-volume-driver shows various ways to mount host sub-directories relative to the compose file.

Answer 1

No because compose/config/config.py#load(config_details) check if datavolume/sql_data matches a named volume (in compose/config/validation.py#match_named_volumes())

datavolume would, datavolume/sql_data would not.

As memetech points out in the comments, the is an issue tracking this since April 2017:
moby/moby issue 32582: "[feature] Allow mounting sub-directories of named volumes".

In that issue, Joohansson adds (see comment)

In the meantime, I use this workaround to mount the whole volume on a separate path and then symlink it to the sub path.

# In the Dockerfile:
RUN mkdir -p /data/subdir
RUN ln -s /data/subdir /var/www/subdir

Then mount the volume as normal.
The /subdir must exist in the volume.

docker run -d -v myvol:/data mycontainer

Now anything read or written by the webserver will be stored in the volume subdir and can't access the other data.