I don’t know which compose file are you talking about. As far as I know there is no “official” compose file.
If you are talking about this mailpile-docker:
then user accounts are on the directory
accounts/ (on the host).
You can use the command
ds gpg <user> [<gpg-commands-and-options>...]
to import/export keys, etc.
The definition of this command is here:
It seems that the GNUPGHOME of a user is located at:
So, you can also manage the GPG keys from the host, finding them at:
P.S. Sorry, I am not allowed to post links on this forum.