aspire-samples tests are failing on latest docker update

[eerhardt]

See the conversation in dotnet/aspire-samples#1480 (comment)

With the latest Docker version, the aspire-samples tests are failing with errors like:

    [2026-02-21 00:02:24Z] info: DatabaseContainers.AppHost.Resources.mysql[0]
      1: 2026-02-21T00:02:24.1720000Z invalid argument "type=volume,src=,target=/var/lib/mysql" for "--mount" flag: invalid value for 'src': value is empty
[2026-02-21 00:02:24Z] info: DatabaseContainers.AppHost.Resources.sqlserver[0]
      1: 2026-02-21T00:02:24.1720000Z invalid argument "type=volume,src=,target=/var/opt/mssql" for "--mount" flag: invalid value for 'src': value is empty
[2026-02-21 00:02:24Z] info: DatabaseContainers.AppHost.Resources.postgres[0]
      1: 2026-02-21T00:02:24.1720000Z invalid argument "type=volume,src=,target=/var/lib/postgresql/data" for "--mount" flag: invalid value for 'src': value is empty

In the tests, we are changing the volume names with the following code:

    public static TBuilder WithRandomVolumeNames<TBuilder>(this TBuilder builder)
        where TBuilder : IDistributedApplicationTestingBuilder
    {
        // Named volumes that aren't shared across resources should be replaced with anonymous volumes.
        // Named volumes shared by mulitple resources need to have their name randomized but kept shared across those resources.

        // Find all shared volumes and make a map of their original name to a new randomized name
        var allResourceNamedVolumes = builder.Resources.SelectMany(r => r.Annotations
            .OfType<ContainerMountAnnotation>()
            .Where(m => m.Type == ContainerMountType.Volume && !string.IsNullOrEmpty(m.Source))
            .Select(m => (Resource: r, Volume: m)))
            .ToList();
        var seenVolumes = new HashSet<string>();
        var renamedVolumes = new Dictionary<string, string>();
        foreach (var resourceVolume in allResourceNamedVolumes)
        {
            var name = resourceVolume.Volume.Source!;
            if (!seenVolumes.Add(name) && !renamedVolumes.ContainsKey(name))
            {
                renamedVolumes[name] = $"{name}-{Convert.ToHexString(RandomNumberGenerator.GetBytes(4))}";
            }
        }

        // Replace all named volumes with randomly named or anonymous volumes
        foreach (var resourceVolume in allResourceNamedVolumes)
        {
            var resource = resourceVolume.Resource;
            var volume = resourceVolume.Volume;
            var newName = renamedVolumes.TryGetValue(volume.Source!, out var randomName) ? randomName : null;
            var newMount = new ContainerMountAnnotation(newName, volume.Target, ContainerMountType.Volume, volume.IsReadOnly);
            resource.Annotations.Remove(volume);
            resource.Annotations.Add(newMount);
        }

        return builder;
    }

The var newName = renamedVolumes.TryGetValue(volume.Source!, out var randomName) ? randomName : null; is the key. We are passing null for a ContainerMountAnnotation src, which used to work but doesn't with the latest Docker update.

@DamianEdwards @karolz-ms @danegsta

[eerhardt]

Here's the repro outside of the aspire-samples:

var builder = DistributedApplication.CreateBuilder(args);

var postgres = builder.AddPostgres("postgres")
    .WithVolume(name: null, "/var/lib/postgresql/data");

builder.Build().Run();

aspire run that app and it fails:

invalid argument "type=volume,src=,target=/var/lib/postgresql/data" for "--mount" flag: invalid value for 'src': value is empty
 
Usage:  docker create [OPTIONS] IMAGE [COMMAND] [ARG...]
 
Run 'docker create --help' for more information

[eerhardt]

@karolz-ms @danegsta - Do we need to update DCP to not generate src= when the name is null?

[karolz-ms]

@eerhardt I think so, yes. Opened an issue on DCP side microsoft/dcp#86 to get that fixed.