FEATURE: Add /containers/new support to serve

[pmusaraj]

Allows another app to trigger a new container via API call.

Example:

curl -N \
    -X POST http://127.0.0.1:7373/containers/new \
    -H "Authorization: Bearer TOKEN" \
    -H "Content-Type: application/json" \
    -d '{
      "name": "core-pr-41785",
      "image": "discourse",
      "pr": "41785"
    }'