This Windows only issue is caused by the missing !n.IsHost() in the n.IsUserDefined() function for windows. Act sends the network alias for the host network on windows, linux sees that and calls the same IsUserDefined (but uses the unix definition on the server side), which detects the host network as non user defined network.

On linux and macOS it is impossible to reproduce at the time of creating this change. I don't get why windows has it's own definition, but bad maintained.

The actual code uses IsUserDefined to detect if network aliases are supported, which didn't work on windows due to the mess in the docker client go library.

Thank you docker client.

go\pkg\mod\github.com\docker\docker@v24.0.7+incompatible\api\types\container\hostconfig_unix.go

// IsUserDefined indicates user-created network
func (n NetworkMode) IsUserDefined() bool {
	return !n.IsDefault() && !n.IsBridge() && !n.IsHost() && !n.IsNone() && !n.IsContainer()
}

go\pkg\mod\github.com\docker\docker@v24.0.7+incompatible\api\types\container\hostconfig_windows.go

// IsUserDefined indicates user-created network
func (n NetworkMode) IsUserDefined() bool {
	return !n.IsDefault() && !n.IsNone() && !n.IsBridge() && !n.IsContainer()
}

Why did docker this?

Seems like we have to replace the convenience function with it's unix definition

Important bug fix, I don't have the power