Fallback image still uses username from failed build

When a Docker image fails to build and the fallback image is used, Envbuilder still attempts to use the username specified in the broken Dockerfile and tries to connect to the fallback image using that username. Often the fallback image doesn’t have the user in the original image and so we cannot connect to the fallback image.

# Reproduction
Dockerfile:
```
FROM mcr.micrdosoft.com/devcontainers/base:ubuntu-22.04
USER vscode
RUN false
```

Fallback image: `ubuntu:24.04`

Should the user be set to `root` when the fallback image is used or even allow a ENVBUILDER_FALLBACK_IMAGE_USER environment variable to be specified?

Start with the Dockerfile reproduction using the shown failed build and fallback image, then trace how Envbuilder selects the user when connecting to the fallback. Done means the fallback image can be used when the original Dockerfile specifies an unavailable user, with the intended root or configurable fallback-user behavior verified.

索引モデルが issue の本文から書いたものです。