Randolph West notes that Microsoft is no longer updating SQL Server images on Docker Hub:
In October 2018, Microsoft announced a change to the source of their Docker containers. You should be using the new Microsoft Container Registry (MCR) as the source for official Docker container images for Microsoft products.
While existing container images in the Docker Hub are not affected, you may not get updated images unless you switch.
On the Docker Hub website, there are instructions on hitting the MCR:
Start a mssql-server instance using the latest update for SQL Server 2017
docker run -e 'ACCEPT_EULA=Y' -e 'SA_PASSWORD=yourStrong(!)Password' -p 1433:1433 -d mcr.microsoft.com/mssql/server:2017-latest