I set my appdata share to Prefer and tried a run with setting it to only. Both exhibit the same behavior.
Here is my Docker settings page
And here is my Assetto template. Is this the docker template you're asking for?
On a retry run, I think this is the furthest I've seen the "download" go. I'll kick off some WireShark and see if I can spot anything out of the ordinary. Network wise, the server has worked perfectly in all other applications.
[ 0%] Checking for available update...
[----] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (329 of 57,309 KB)...
[ 0%] Downloading update (569 of 57,309 KB)...
[ 0%] Downloading update (721 of 57,309 KB)...
[ 1%] Downloading update (1,445 of 57,309 KB)...
[ 2%] Downloading update (2,115 of 57,309 KB)...
[ 3%] Downloading update (2,984 of 57,309 KB)...
[ 5%] Downloading update (3,995 of 57,309 KB)...
[ 6%] Downloading update (5,153 of 57,309 KB)...
[ 8%] Downloading update (6,233 of 57,309 KB)...
[ 10%] Downloading update (0 of 57,309 KB)...
[----] Verifying installation...
[ 0%] Downloading Update...
[ 0%] Checking for available update...
[----] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] Downloading update (570 of 57,309 KB)...
[ 0%] Downloading update (370 of 57,309 KB)...
[ 0%] Downloading update (0 of 57,309 KB)...
[ 0%] !!! Fatal Error: Steamcmd needs to be online to update. Please confirm your network connection and try again.