devcontainer.json does not honor configuration when containerUser is not set

[alexveecle]

We've found an issue where devcontainer.json does not work unless you force a containerUser.

I believe this is similar to #457.

I think this problem stems from:

• When UserFromDockerfile does not find the user in the Dockerfile, it calls userFromImage to find it.
• This uses the same code that Unauthenticated request error installing feature from private oci registry #457 describes.

In our case, in development environments we set ENVBUILDER_INSECURE to be able to use registries without proper TLS, but with this issue, that does not work.

Our workaround is to set containerUser for now and avoid the issue.