Cirrus-ci-docs: Support mapping ports on additional_containers

[mattlorey]

Description

When starting multiple additional_containers, be able to map the port

Context

We want to be able to run two postgres instances in tests, but cannot since they both bind to port 5432

Anything Else

Specify the local port and the port exposed in the container

    additional_containers:
      - name: postgres
        image: postgres:alpine
        port: 5432:5432
        env:
          ...
      - name: postgres
        image: postgres:alpine
        cpu: 0.5
        memory: 1G
        port: 5433:5432
        env:
          ...

[fkorotkov]

Made the change internally and will update documentation once the schema update will be deployed and it will become available for everyone. 🙌This will take a bit longer then implementing the actual change 😅