Use the startup callback to write the my.cnf configuration file

src/Testcontainers.MySql/MySqlBuilder.cs

@@ -89,7 +89,8 @@ protected override MySqlBuilder Init()
             .WithPortBinding(MySqlPort, true)
             .WithDatabase(DefaultDatabase)
             .WithUsername(DefaultUsername)
-            .WithPassword(DefaultPassword);
+            .WithPassword(DefaultPassword)
+            .WithStartupCallback((container, ct) => container.WriteConfigurationFile(ct));
     }
 
     /// <inheritdoc />

src/Testcontainers.MySql/MySqlContainer.cs

@@ -5,7 +5,6 @@ namespace Testcontainers.MySql;
 public sealed class MySqlContainer : DockerContainer, IDatabaseContainer
 {
     private readonly MySqlConfiguration _configuration;
-    private bool _hasConfigFile;
 
     /// <summary>
     /// Initializes a new instance of the <see cref="MySqlContainer" /> class.
@@ -46,20 +45,23 @@ public async Task<ExecResult> ExecScriptAsync(string scriptContent, Cancellation
         await CopyAsync(Encoding.Default.GetBytes(scriptContent), scriptFilePath, Unix.FileMode644, ct)
             .ConfigureAwait(false);
 
-        if (!_hasConfigFile)
-        {
-            var config = new StringWriter { NewLine = "\n" };
-            config.WriteLine("[client]");
-            config.WriteLine("protocol=TCP");
-            config.WriteLine($"user={_configuration.Username}");
-            config.WriteLine($"password={_configuration.Password}");
-            await CopyAsync(Encoding.Default.GetBytes(config.ToString()), "/etc/mysql/my.cnf", UnixFileModes.UserRead | UnixFileModes.UserWrite, ct)
-                .ConfigureAwait(false);
-
-            _hasConfigFile = true;
-        }
-
         return await ExecAsync(new[] { "mysql", _configuration.Database, $"--execute=source {scriptFilePath};" }, ct)
             .ConfigureAwait(false);
     }
+
+    /// <summary>
+    /// Writes a configuration file so that calling <see cref="ExecScriptAsync"/> does not
+    /// produce a warning on stderr about using a password on the command line.
+    /// </summary>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>Task that completes when the configuration file has been written.</returns>
+    internal Task WriteConfigurationFile(CancellationToken ct)
+    {
+        var config = new StringWriter { NewLine = "\n" };
+        config.WriteLine("[client]");
+        config.WriteLine("protocol=TCP");
+        config.WriteLine($"user={_configuration.Username}");
+        config.WriteLine($"password={_configuration.Password}");
+        return CopyAsync(Encoding.Default.GetBytes(config.ToString()), "/etc/mysql/my.cnf", UnixFileModes.UserRead | UnixFileModes.UserWrite, ct);
+    }
 }

src/Testcontainers/Builders/ContainerBuilder`3.cs

@@ -348,9 +348,9 @@ public TBuilderEntity WithWaitStrategy(IWaitForContainerOS waitStrategy)
     }
 
     /// <inheritdoc />
-    public TBuilderEntity WithStartupCallback(Func<IContainer, CancellationToken, Task> startupCallback)
+    public TBuilderEntity WithStartupCallback(Func<TContainerEntity, CancellationToken, Task> startupCallback)
     {
-      return Clone(new ContainerConfiguration(startupCallback: startupCallback));
+      return Clone(new ContainerConfiguration(startupCallback: (container, ct) => startupCallback((TContainerEntity)container, ct)));
     }
 
     /// <inheritdoc />

src/Testcontainers/Builders/IContainerBuilder`2.cs

@@ -425,6 +425,6 @@ public interface IContainerBuilder<out TBuilderEntity, out TContainerEntity> : I
     /// <param name="startupCallback">The callback method to invoke.</param>
     /// <returns>A configured instance of <typeparamref name="TBuilderEntity" />.</returns>
     [PublicAPI]
-    TBuilderEntity WithStartupCallback(Func<IContainer, CancellationToken, Task> startupCallback);
+    TBuilderEntity WithStartupCallback(Func<TContainerEntity, CancellationToken, Task> startupCallback);
   }
 }