Impossibile connettersi a SQL Server da un contenitore Docker

Abbiamo creato l'app ASP NET Core con il server sql esistente.

const string connectionString = @"Server=hostname\sqlexpress;Database=DEVDB;User Id=sa;Password=password;";
services.AddDbContext<RecruitmentToolsDatabaseContext>(options => options.UseSqlServer(connectionString));

Funziona su Windows. Dopo aver creato l'immagine della finestra mobile con le stesse impostazioni non possiamo connetterci al server db.

L'errore:

fail: Microsoft.EntityFrameworkCore.Query.Internal.SqlServerQueryCompilationContextFactory[1]
    An exception occurred in the database while iterating the results of a query.
    System.Data.SqlClient.SqlException: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections.(provider: TCP Provider, error: 35 - An internal exception was caught) ---> System.AggregateException: One or more errors occurred. (No such device or address) ---> System.Net.Internals.SocketExceptionFactory+ExtendedSocketException: No such device or address
        at System.Net.Dns.HostResolutionEndHelper(IAsyncResult asyncResult)
        at System.Net.Dns.EndGetHostAddresses(IAsyncResult asyncResult)
        at System.Net.Dns.<>c.<GetHostAddressesAsync>b__14_1(IAsyncResult asyncResult)
        at System.Threading.Tasks.TaskFactory`1.FromAsyncCoreLogic(IAsyncResult iar, Func`2 endFunction, Action`1 endAction, Task`1 promise, Boolean requiresSynchronization)
    --- End of stack trace from previous location where exception was thrown ---

Abbiamo creato la configurazione freetds /etc/freetds/freetds.conf

[dev01]
host = hostname 
instance = sqlexpress
tds version = 8.0" 

e siamo stati in grado di connetterci a quel db usando sqsh.

Qualche idea?

modificare

Il mio file Docker:

FROM microsoft/dotnet:latest
COPY . /app
WORKDIR /app

# Install apt packages
RUN apt-get update && apt-get install -y \
    freetds-bin \
    freetds-common \
    freetds-dev \
    sqsh

RUN touch /etc/freetds/freetds.conf
RUN echo "[dev01]" > /etc/freetds/freetds.conf
RUN echo "host = hostname" >> /etc/freetds/freetds.conf
RUN echo "instance = sqlexpress" >> /etc/freetds/freetds.conf
RUN echo "tds version = 8.0" >> /etc/freetds/freetds.conf

RUN touch /home/.sqshrc
RUN echo "\set username=sa" >  /home/.sqshrc
RUN echo "\set password=password" >>  /home/.sqshrc
RUN echo "\set style=vert" >>  /home/.sqshrc

RUN ["dotnet", "restore"]
RUN ["dotnet", "build"]

EXPOSE 5000/tcp
ENV ASPNETCORE_URLS http://*:5000

ENTRYPOINT ["dotnet", "run"]
#CMD ["bash"] 

Oltre al normale ASP .NET Core questa immagine contiene freetds e sqsh per il nostro scopo di test. La nostra app si avvia normalmente e sta funzionando. Il problema si verifica quando proviamo a connetterci a sql getta l'eccezione .net.