Cannot connect to custom SQL Server docker image - sql-server

Question

we're currently using Windows containers and, as there are no official SQL Server images for Windows, we are building our own.
We've successfully created the image and by running via
docker run --name SQLServer2022preview -e ACCEPT_EULA=Y -e sa_password=PaSSw0rd -p 1433:1433 -d mycompany:winsrv1809-sql2022preview
I can connect and work as expected to localhost,1433. However, this image will act as a base and we need to build another image based on this on runtime. When doing so, is where it can't connect. We're getting this message
Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server : Named Pipes Provider: Could not open a connection to SQL Server [2]. .
Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server : Login timeout expired.
Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server : A network-related or instance-specific error has occurred while establishing a connection to SQL Server. Server is not found or not accessible. Check if instance name is correct and if SQL Server is configured to allow remote connections. For more information see SQL Server Books Online..
The command 'powershell -Command $ErrorActionPreference = 'Stop'; $ProgressPreference = 'SilentlyContinue'; sqlcmd -i "C:\restore.sql"' returned a non-zero code: 1
This is the docker file
FROM mycompany:winsrv1809-sql2022preview
EXPOSE 1433
EXPOSE 4338
COPY ./databases/DB1.bak C:/
COPY ./HAS_Database/restore.sql C:/
RUN sqlcmd -i "C:\\restore.sql"
and restore.sql
RESTORE DATABASE DB1
FROM DISK = N'C:\DB1.bak'
WITH FILE = 1,
MOVE N'DB1_Imported'
TO N'C:\DB1.mdf',
MOVE N'DB1_log'
TO N'C:\DB1_log.ldf',
NOUNLOAD,
STATS = 5;
GO
There's no login errors in sql log looking into the container (which automatically closes after aprox 30 sec) so I guess that it may be relative to port forwarding. Can anyone help?
I tried port forwarding in the start step, with no luck. I'm a docker newbie