Got it -- just in case someone else has the same issue:

When I pulled from the command line, it appears to have retrieved a newer image.

1. SSH to Synology NAS.
2. Change directory to /volume1/@docker
3. Run command: docker pull [mcr.microsoft.com/dotnet/core/sdk](https://mcr.microsoft.com/dotnet/core/sdk)
4. Wait; when finished you can type: docker images to see all available images.
5. Using the Synology Docker interface to start it.
6. Profit

Reaching out for assistance -- I've had a docker image that allows me to run dotnet core 2.0 console apps running for a long time now, but whenever I try to get a new image that would support dotnet core 3, I get a new image that only runs 2.0.  Does anyone have this working?

DotNet Core 3.0?