What is the problem you're trying to solve
"init containers" is a common and useful pattern to implement environment setup for a service container.
There's no such concept in the Compose model

Describe the solution you'd like
Actual syntax to be debated, but I assume init container basically have the same configuration as a service container, but

• implicitly inherits volumes (explicit and anonymous) from service
• inherits deploy

services:
  myapp:
    image: TBD
    init:
      - image: alpine/openssl
        command: ecparam -noout -genkey -name prime256v1 -out /opt/private_key
      - image: alpine/curl
        command: -X POST http://xxx/register'

Additional context
This is a follow up for #11