`containerEnv` input validation

Debugged a fun issue recently with a `containerEnv` similar to below:

```jsonc
"containerEnv": {
"BUNDLE_RUBYGEMS__PKG__GITHUB__COM`": "${localEnv:GITHUB_USER}:${localEnv:GITHUB_TOKEN}"
}
```

Notice the stray backtick character at the end of the environment key 😅

Such an environment key causes a failure when [setting up dotfiles](https://github.com/devcontainers/cli/blob/main/src/spec-common/dotfiles.ts#L74).

The CLI would benefit from additional input validation to make such typos easier to debug.