Multi-word containerEnv variables for features broken

[MihaiBSony]

Passing a multi-word containerEnv variable to a feature does not work without explicitly adding quotes. For example the following devcontainer-feature.json:

{
  ...
  "containerEnv": {
    "MY_VAR": "some value"
  }
}

Leads to the dockerfile command ENV MY_VAR=some value which is invalid, since the ENV command allows multiple key-value pairs. Instead, the user currently has to use "MY_VAR": "\"some value\"".

[bamurtaugh]

cc @joshspicer