I ran into a problem when updating my pihole container last week - the container initialization process creates a new, randomized, password for the admin account used for web administration! Because I run it as a container, there’s no output on first boot to let me see the password, and it’s not stored anywhere in the pihole server logs or configuration.

Fortunatey, Docker makes it very easy to find the password. On the server hosting the docker instance, run
docker logs pihole | grep random

After doing a bit of research, it turns out that I can set the FTLCONF_webserver_api_password variable in my configuration file to specify the admin password that I want. I’ve set it this way now, which should hopefully make running (and upgrading) pihole even more autonomous.

References

Some articles I referenced when figuring this out were:

Configuration - Pi-hole documentation
https://www.reddit.com/r/pihole/comments/1277bhm/how_do_i_find_my_webgui_password_after_installing/