Default Values are not added into .env File DockerComposeEnvironment

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Describe the bug

When using
```
builder.AddDockerComposeEnvironment("compose")
.ConfigureEnvFile(env =>
{
env["CUSTOM_VAR"] = new()
{
Name = "CUSTOM_VAR",
DefaultValue = "my-value"
};
});
```

the output .env file contains the environment parameter without defaultValue
```
CUSTOM_VAR=
```

### Expected Behavior

I would expect that the .env file contains
```
CUSTOM_VAR=my-value
```

### Steps To Reproduce

Create a new Project
`aspire new aspire-starter -n AspireApp -o AspireApp`
Add Package Reference
``

change AppHost.cs to
```
var builder = DistributedApplication.CreateBuilder(args);

// var apiService = builder.AddProject("apiservice")
// .WithHttpHealthCheck("/health");

// builder.AddProject("webfrontend")
// .WithExternalHttpEndpoints()
// .WithHttpHealthCheck("/health")
// .WithReference(apiService)
// .WaitFor(apiService);

builder.AddDockerComposeEnvironment("compose")
.ConfigureEnvFile(env =>
{
env["CUSTOM_VAR"] = new()
{
Name = "CUSTOM_VAR",
DefaultValue = "my-value"
};
});

builder.Build().Run();
```

Run
`dotnet aspire publish`

### Exceptions (if any)

_No response_

### .NET Version info

.NET SDK:
Version: 10.0.101
Commit: fad253f51b
Workload version: 10.0.100-manifests.9f71effe
MSBuild version: 18.0.6+fad253f51

Laufzeitumgebung:
OS Name: Windows
OS Version: 10.0.26100
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\10.0.101\

Installierte .NET-Workloads:
Es sind keine installierten Workloads zum Anzeigen vorhanden.
Für die Verwendung von workload sets bei der Installation neuer Manifeste konfiguriert.
Es sind keine Workloadsätze installiert. Führen Sie „dotnet workload restore“ aus, um einen Workloadsatz zu installieren.

Host:
Version: 10.0.1
Architecture: x64
Commit: fad253f51b

.NET SDKs installed:
8.0.418 [C:\Program Files\dotnet\sdk]
9.0.205 [C:\Program Files\dotnet\sdk]
9.0.308 [C:\Program Files\dotnet\sdk]
9.0.311 [C:\Program Files\dotnet\sdk]
10.0.101 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.24 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.6 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 10.0.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.24 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.6 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 10.0.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 7.0.20 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.22 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.24 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.6 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 9.0.13 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 10.0.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
x86 [C:\Program Files (x86)\dotnet]
registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
DOTNET_CLI_TELEMETRY_OPTOUT [true]

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download

### Anything else?

_No response_