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

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

I believe this is similar to .

I think this problem stems from:

* When `UserFromDockerfile` [does not find the user in the `Dockerfile`](https://github.com/coder/envbuilder/blob/55a0ba3515b70edb6d066e5dd5944c9805d9ecb2/devcontainer/devcontainer.go#L373), it calls [`userFromImage`](https://github.com/coder/envbuilder/blob/55a0ba3515b70edb6d066e5dd5944c9805d9ecb2/devcontainer/devcontainer.go#L437) to find it.
* This uses the same code that #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.