Fallback not working when invalid Dockerfile in coder workspace

When I use devcontainer in coder (kubernetes) from this template:

[template_workspace_coder.txt](https://github.com/user-attachments/files/17175483/template_workspace_coder.txt)

And change the Dockerfile to this and restart:
Dockerfile:
INVALID
FROM qmcgaw/godevcontainer

Then envbuilder is not detecting the failure and is not reverting to the fallback image


Reproduce the failure with the attached template in a coder Kubernetes workspace, changing the Dockerfile to begin with INVALID before restarting. Trace envbuilder's Dockerfile failure handling and fallback-image selection. Done means the invalid Dockerfile failure is detected and the workspace reverts to the fallback image.

Written by the indexing model from the issue text.