Black screen when starting on pmOS/Alpine

I'm working on porting phog to Alpine/pmOS on the Librem5. I'm able to build the package and install it on the device.

I can then start greetd with

sudo usermod -a -G video greetd
sudo service greetd start

Setting the line in /etc/greetd/config.toml to:

command = "cage -s -- gtkgreet"

brings up the gtkgreet test and it appears to work correctly.

If I change command to

command = "phog"

I just get a black screen with a cursor. Sometimes I manage to see an eglQueryDeviceString EGL_BAD_PARAMETER error, but that seems to be this unrelated issue: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3275

The logs in logread are

authpriv.info : pam_unix(greetd:session): session opened for user greetd(uid=111) by (uid=0)
auth.warn elogind[2515]: Failed to create inotify watch on /dev/null/utmp, ignoring: Not a directory
auth.info elogind[2515]: New session c7 of user greetd.
authpriv.info : gkr-pam: gnome-keyring-daemon started properly

I'm using wlroots 0.17 and phoc 0.33, so they should be new enough to work with the 0.1.6 release.

Any ideas how how to get more debug information or how to get this working?