Hooray!!! Success!!!
Here's what I did:
I had a little trouble deleting my Valheim folder from APPDATA but I got it done via the Unraid console (I am not a UNIX guy 🙂 so I had to look up the commands.)
Then I installed again and, with the clean install, I copied my single player world files into the "worlds" directory for the docker.
I left the docker template mostly the same. I only added these things:
GAME_PARAMS = -console so I can use the manual save feature (this is not work, FYI)
Server Name = [my server name]
World Name = [my world name]
Automatically Update Game = false
Backups to keep = 12
That's all I changed.
I immediately saw the server in the Steam Servers list.
I launched from Steam and - within Valheim - went to the Join Community tab.
I hit Refresh for the server list and waited for all 9000+ servers to load.
Then I used the filter to find mine, but it didn't appear in the list.
I tried this multiple times but after 30 minutes the server still didn't appear in Valheim's list.
So I clicked Join IP, entered the IP address and port, and hit ENTER.
It joined right away and displayed that weird password screen (the one without any buttons).
I entered the password, pressed the REAL ENTER key (not that fake key on the NUMPAD).
Boom! I'm in! I'm so happy we can play together or separately now.
Thanks, @ich777 for another stellar solution.