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: https://gitlab.com/docker-scripts/mailpile
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:
https://gitlab.com/docker-scripts/mailpile/blob/master/cmd/gpg.sh
It seems that the GNUPGHOME of a user is located at: "/home/<user>/.local/share/Mailpile/default"
So, you can also manage the GPG keys from the host, finding them at: "accounts/<user>/.local/share/Mailpile/default"
P.S. Sorry, I am not allowed to post links on this forum.