The default password is "Docker" (without quotes).
You have to enable ValheimPlus in the template, start the container once with a password and wait for it to start up completely after that stop the container.
Go to your Valheim directory, edit the file ".../BepInEx/config/valheim_plus.cfg", search for the string "disableServerPassword", set it to "true" (without quotes) and save the file.
After you did that go to the Valheim template, turn on Advanced View on the top right corner and click on Edit at the Password (don't delete it), at the drop down for Required set it so "No", delete the Value, and click Save and Apply:
Please note that only players with ValheimPlus can connect if you've enabled ValheimPlus from what I know, maybe this can be also disabled in the config but this is a thing that you have to research on your own.