Support env variables for YAML file

We're currently exposing our AWS account number through the YAML file. It would be helpful to inject this via env variables:

**Actual:**
```
apache:
  build: Dockerfile
  image: 123456789.dkr.ecr.ap-southeast-2.amazonaws.com/foobar/apache
```

**Expected:**
```
apache:
  build: Dockerfile
  image: $DOCKER_HUB_DOMAIN/foobar/apache
```