I do not know if this is the same issue as you have. but is is the procedure to go into the sqlite database.
I also tried the procedure I found on the nginx pm site.
You need to install sqlite in the NPM container
For apline the procedure is al little bit different as described in the above URL.
Login to the NPM commandline via putty.
docker ps | grep nginx (find the container-name)
docker exec -it <container-name> sh
apk update
apk add --no-cache sqlite
sqlite3 /config/database.sqlite
You have now entered the SQL mode, where you set the status of all users to deleted:
UPDATE user SET is_deleted=1;
.exit
exit
When you have doe this you have to restart the container or in other words the add-on.
The first login will be with the default credentials again:
login: admin@example.com
pass: changeme