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!