Connect to SQL server container with other docker container

I have a Docker container with SQL Server:

sudo docker run -e "ACCEPT_EULA=Y" -e "SA_PASSWORD=Foo" \
   -p 1433:1433 --name sql1 \
   -d mcr.microsoft.com/mssql/server:2019-GA-ubuntu-16.04

I want to connect to this SQL Server from my application, using this connection string:

"DefaultConnection": "Server=sql1,1433;Database=mydb;User Id=superuser;Password=Foo;TrustServerCertificate=True",

My docker container which wants to connect is run with

sudo docker run -d -t --name app --network=mynetwork -p 911:80 app app;

But when I run this, I get an error:

Unhandled exception. System.AggregateException: One or more errors occurred. (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: 40 - Could not open a connection to SQL Server))

Microsoft.Data.SqlClient.SqlException (0x80131904): 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: 40 - Could not open a connection to SQL Server)

at Microsoft.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, SqlCredential credential, Object providerInfo, String newPassword, SecureString newSecurePassword, Boolean redirectedUserInstance, SqlConnectionString userConnectionOptions, SessionData reconnectSessionData, Boolean applyTransientFaultHandling, String accessToken, DbConnectionPool pool, SqlAuthenticationProviderManager sqlAuthProviderManager) at Microsoft.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, DbConnectionPoolKey poolKey, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection, DbConnectionOptions userOptions) at Microsoft.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnectionPool pool, DbConnection owningObject, DbConnectionOptions options, DbConnectionPoolKey poolKey, DbConnectionOptions userOptions) at Microsoft.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection) at Microsoft.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject, DbConnectionOptions userOptions, DbConnectionInternal oldConnection) at Microsoft.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, UInt32 waitForMultipleObjectsTimeout, Boolean allowCreate, Boolean onlyOneCheckConnection, DbConnectionOptions userOptions, DbConnectionInternal& connection) at Microsoft.Data.ProviderBase.DbConnectionPool.TryGetConnection(DbConnection owningObject, TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal& connection) at Microsoft.Data.ProviderBase.DbConnectionFactory.TryGetConnection(DbConnection owningConnection, TaskCompletionSource1 retry, DbConnectionOptions userOptions, DbConnectionInternal oldConnection, DbConnectionInternal& connection) at Microsoft.Data.ProviderBase.DbConnectionInternal.TryOpenConnectionInternal(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 retry, DbConnectionOptions userOptions) at Microsoft.Data.ProviderBase.DbConnectionClosed.TryOpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory, TaskCompletionSource1 retry, DbConnectionOptions userOptions) at Microsoft.Data.SqlClient.SqlConnection.TryOpen(TaskCompletionSource1 retry) at Microsoft.Data.SqlClient.SqlConnection.Open() at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.OpenDbConnection(Boolean errorsExpected) at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.Open(Boolean errorsExpected) at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerDatabaseCreator.<>c__DisplayClass18_0.<Exists>b__0(DateTime giveUp) at Microsoft.EntityFrameworkCore.ExecutionStrategyExtensions.<>c__DisplayClass12_02.b__0(DbContext c, TState s) at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.Execute[TState,TResult](TState state, Func3 operation, Func3 verifySucceeded) at Microsoft.EntityFrameworkCore.ExecutionStrategyExtensions.Execute[TState,TResult](IExecutionStrategy strategy, Func2 operation, Func2 verifySucceeded, TState state) at Microsoft.EntityFrameworkCore.ExecutionStrategyExtensions.Execute[TState,TResult](IExecutionStrategy strategy, TState state, Func`2 operation) at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerDatabaseCreator.Exists(Boolean retryOnNotExists) at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerDatabaseCreator.Exists() at Microsoft.EntityFrameworkCore.Migrations.HistoryRepository.Exists() at Microsoft.EntityFrameworkCore.Migrations.Internal.Migrator.Migrate(String targetMigration) at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.Migrate(DatabaseFacade databaseFacade) at App.Data.ContextSeed.SeedAsync(IServiceProvider services) in /src/Data/ContextSeed.cs:line 36 ClientConnectionId:00000000-0000-0000-0000-000000000000 --- End of inner exception stack trace --- at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken) at System.Threading.Tasks.Task.Wait() at App.Program.Main(String[] args) in /src/App/Program.cs:line 26

The network is defined as follows:

[
    {
        "Name": "mynetwork",
        "Id": "2835e7316239a6dfdd031e6d45b9ae4dc173bcc643312d4446ec507ec6147cb6",
        "Created": "2019-12-22T18:56:38.588203724+01:00",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": {},
            "Config": [
                {
                    "Subnet": "172.18.0.0/16",
                    "Gateway": "172.18.0.1"
                }
            ]
        },
        "Internal": false,
        "Attachable": false,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "4196c4b6221699a7f0e925313ce1f06b982b8b0b121c1b68e14392d55bd4ca33": {
                "Name": "sql1",
                "EndpointID": "8c32840cc53c5c2cdee29fb71b7ad2bcd69e90df6b9cc9e4153235665743aa23",
                "MacAddress": "02:42:ac:12:00:02",
                "IPv4Address": "172.18.0.2/16",
                "IPv6Address": ""
            }
        },
        "Options": {},
        "Labels": {}
    }
]

Since the app doesn't start, it is not visible in the network.

What am I doing wrong?

Good to know:

• Connecting with SSMS from a windows PC works fine
• Docker here runs on Linux
• Linux Ubuntu 18.04
• DotNet 3.1.100

EDIT:

Even more weird, NLog does output to the database.

Nlog.config

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      autoReload="true"
      internalLogLevel="Info">

  <!-- enable asp.net core layout renderers -->
  <extensions>
    <add assembly="NLog.Web.AspNetCore"/>
  </extensions>

  <!-- the targets to write to -->
  <targets>
    <target name="database"
           xsi:type="Database"
           connectionString="${appsettings:name=ConnectionStrings.DefaultConnection}"
           commandText="insert into dbo.Log (
                  MachineName, Logged, Level, Message,
                  Logger, Callsite, Exception
                ) values (
                  @MachineName, @Logged, @Level, @Message,
                  @Logger, @Callsite, @Exception
                );">
      <parameter name="@MachineName" layout="${machinename}" />
      <parameter name="@Logged" layout="${date}" />
      <parameter name="@Level" layout="${level}" />
      <parameter name="@Message" layout="${message}" />
      <parameter name="@Logger" layout="${logger}" />
      <parameter name="@Callsite" layout="${callsite}" />
      <parameter name="@Exception" layout="${exception:tostring}" />
    </target>
  </targets>

  <!-- rules to map from logger name to target -->
  <rules>
    <logger name="*" minLevel="Trace" writeTo="database"/>

    <!--All logs, including from Microsoft-->
    <logger name="*" minlevel="Trace" writeTo="logconsole" />

    <!--Skip non-critical Microsoft logs and so log only own logs-->
    <logger name="Microsoft.*" maxlevel="Info" final="true" />
    <!-- BlackHole without writeTo -->
    <logger name="*" minlevel="Trace" writeTo="ownFile-web" />

  </rules>
</nlog>

Blocking part of the context seed:

    public static class ContextSeed
    {
        private static Boolean? useInMemory = null;

        public static void Init(Boolean _useInMemory = false)
        {
            useInMemory = _useInMemory;
        }

        public static async Task SeedAsync(IServiceProvider services)
        {
            var context = services.GetRequiredService<ApplicationDbContext>();
            var userManager = services.GetRequiredService<UserManager<User>>();
            var roleManager = services.GetRequiredService<RoleManager<IdentityRole<String>>>();

            if (useInMemory == null)
            {
                throw new ApplicationException("Please initialize the ContextSeed using the Init method.");
            }

            if (!useInMemory.Value)
            {
                context.Database.Migrate();
            }
        }
     }

Creation of db context as per request:

            services.AddDbContext<ApplicationDbContext>(options =>
            {
                ContextSeed.Init();
                options.UseSqlServer(
                    Configuration.GetConnectionString("DefaultConnection"));
            });