Fallback image on "error: compile devcontainer.json"

I accidentally broke my devcontainer, which gives this error and a crashloop:
```
error: compile devcontainer.json: user from dockerfile: parse image ref "python:$PY_VERSION": could not parse reference: python:$PY_VERSION
```
I know this is #455 but it would be nice if it just used the fallback_image if the devcontainer doesn't compile. That way I can edit the devcontainer in coder and don't have to go into the pod manually or edit it on the github website.
Thanks y'all!

Start by tracing the devcontainer compilation error path and the existing fallback_image handling. Reproduce the malformed python:$PY_VERSION case, then verify that a compile failure launches the fallback image and leaves the environment usable for editing in Coder.

Written by the indexing model from the issue text.