Prior to this commit, the docker compose driver would _always_ path
expand a host volume no matter what. This is not always the correct
option, for example if that host volume is actually a reference to a key
inside a `volumes` hash instead of a path on disk. This commit changes
that by looking to see if the requested host volume is actually a
defined key inside the compose config, and if not, it will path expand
it like before. Otherwise it will leave the key "as is".
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