Multi-machine not working in production

**Description**
Multi-machine works with the configuration in `docker-compose-multi-machine.yml` (development) but not with the configuration in `release/docker-compose-hub-image.yml` (production).
In production the machines are started but they are not connected to the lab.
Possible causes of this issue:
1. The configured port in `hub-image.yml` (MACHINE_PORT=5081) needs to match the exposed docker ports.
2. The `hub-image.yml` file is missing the MACHINE_HOST var (as in the `multi-machine.yml` file)
3. A combination of 1 and 2

These possibilities need to be troubleshooted. `hub-image.yml` needs to be updated with the correct parameters.

**Steps to reproduce**
1. Download the latest Aliro-*.zip (v0.21.1) from the GitHub releases page.
2. Uncomment the lines for "additional machine containers"
3. Run `docker compose up`
4. The multiple machines are started, but Aliro only submits experiments to the first machine.