Run `devenv shell` when creating the container

[domenkozar]

We'd want to support Docker and Podman, the two most popular desktop container engines.

The only compatibility layer that exists is Docker RESTful API, where the workflow would be something like:

1. Build our container

curl -X POST http://localhost:2375/containers/create \
     -H "Content-Type: application/json" \
     -d '{
           "Image": "spec.json",
           "Cmd": ["devenv", "shell"],
         }'

3. curl -X POST http://localhost:2375/containers/{id}/start

curl -X POST "http://localhost:2375/commit?container={id}" \
     -H "Content-Type: application/json" \
     -d '{
           "repo": "mynewimage",
           "tag": "latest",
           "comment": "This is my new image",
           "author": "Your Name",
         }'

I'm not sure yet what this would be called, but for now I propose containers.<name>.impureShell = "...";.

cc @mcdonc