Document advanced use cases for GitLab CI/CD and private container image registries

* [ ] AWS instance profiles as per https://github.com/iterative/terraform-provider-iterative/issues/602#issuecomment-1145964625
* [ ] Configuration as part of the workflow variables, as in e.g.
```yaml
variables:
AWS_ACCOUNT: 000000000000
AWS_DEFAULT_REGION: us-east-1
AWS_REGISTRY: ${AWS_ACCOUNT}.dkr.ecr.${AWS_DEFAULT_REGION}.amazonaws.com
AWS_CONTAINER_IMAGE: ${AWS_REGISTRY}/example:latest
DOCKER_AUTH_CONFIG: '{"credHelpers": {"${AWS_REGISTRY}": "ecr-login"}}'

build:
rules:
- changes: [Dockerfile, .gitlab-ci.yml]
services:
- docker:dind
image: docker
script:
- docker run --env AWS_ACCESS_KEY_ID --env AWS_SECRET_ACCESS_KEY --env AWS_SESSION_TOKEN --env AWS_DEFAULT_REGION amazon/aws-cli ecr get-login-password |
docker login --username AWS --password-stdin ${AWS_REGISTRY}
- docker build --tag ${AWS_CONTAINER_IMAGE} .
- docker push ${AWS_CONTAINER_IMAGE}

launch:
image: iterativeai/cml:0-dvc2-base1
script: >
cml runner launch
--cloud=aws
--cloud-region=$AWS_DEFAULT_REGION
--cloud-type=m5.2xlarge
--cloud-hdd-size=32
--labels=cml

train:
needs: [launch]
tags: [cml]
image: ${AWS_CONTAINER_IMAGE}
script:
- echo hello world
```