If you've already created a Docker container with something like SQL Server 2019 Developer edition:
$ docker run -e ACCEPT_EULA=Y -e MSSQL_PID=Developer -e MSSQL_SA_PASSWORD=YourStrongPassw0rd -p 1433:1433 -d mcr.microsoft.com/mssql/server:2019-latestWith reference to Configure SQL Server on Linux with the mssql-conf tool, get a root shell in the container:
$ docker exec -it --user root 57301203bac5 bash