update tianon/mutt-gmail with a proper $HOME, GPG support (just bind-mount your .gnupg folder to /root/.gnupg and optionally set GPG_ID to the key ID you want to sign emails with), and some better "gmail" settings

also, if you create /root/.muttrc.local (via bind-mount or a new Dockerfile with ADD), it'll be sourced at the end of muttrc :)