Wouldn't it work if you did something like HEX_HOME=${srcdir}/.hex MIX_HOME=${srcdir}/.mix MIX_ENV=prod mix do phx.digest, release --overwrite --path=${srcdir}/usr/share/webapps/${pkgname} or whatever other path and then move the files in package()?
Obviously I haven't tried it, but it might be simplest solution.
I see that in first incarnation this PKGBUILD used this mix do command both in build and in package.

Hello! mix do phx.digest requires $pkgdir folder, which in turn appears at the stage of package().

Hello, could you move the compile/mix step to build() instead of package()? I think it would make more sense. Thanks

I fixed all needed (as I think). Please check you email, I send you proposed config.

1. maybe change a lot of environments from .service to one EnvironmentFile? For example: EnvironmentFile=/etc/teslamate.conf

1. maybe move teslamate from /opt to /usr/share/webapps, like in other packages?
2. in /var/lib/grafana/conf/provisioning/dashboards/teslamate.yml 3 providers, this is wrong and grafana logs have a lot of warnings. Needed only one provider: name: 'teslamate'

What is the recommended way to set the ENCRYPTION_KEY variable w/o setting it system-wide?