TypeError: Cannot read properties of null (reading 'Env')

Getting this error suddenly from CLI/Vscode/Cursor:
```
➜ devcontainer build --workspace-folder . --config .devcontainer/devcontainer.json --image-name=test
[3 ms] @devcontainers/cli 0.75.0. Node.js v20.12.2. darwin 24.3.0 arm64.
TypeError: Cannot read properties of null (reading 'Env')
at o6 (/Users/jasondamour/.nvm/versions/node/v20.12.2/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:393:20372)
at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
at async y6 (/Users/jasondamour/.nvm/versions/node/v20.12.2/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:467:2352)
at async dw (/Users/jasondamour/.nvm/versions/node/v20.12.2/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:467:1886)
at async R7 (/Users/jasondamour/.nvm/versions/node/v20.12.2/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:667:2203)
at async S7 (/Users/jasondamour/.nvm/versions/node/v20.12.2/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:666:4937)
at async /Users/jasondamour/.nvm/versions/node/v20.12.2/lib/node_modules/@devcontainers/cli/dist/spec-node/devContainersSpecCLI.js:484:1188
{"outcome":"error","message":"Cannot read properties of null (reading 'Env')","description":"An error occurred building the container."}
```

With the dockerfile:
```
FROM mcr.microsoft.com/devcontainers/base:1.2-ubuntu-22.04
```

Given I haven't upgraded the CLI in a while and this was working fine recently, I believe this is caused by a change to the base Docker image. Semi-confirmed by changing my base image to `ubuntu:22.04` and it worked fine.

However, going to use this repo for the issue. If an image doesn't have an `Env` property, shouldn't the CLI still succeed?