fix: yaml.parser.ParserError due to unrendered image name in dev

[regisb]

A user reported the following error on tutor dev start:

======================================================
    Starting the platform in detached mode
======================================================
docker-compose -f /home/ubuntu/.local/share/tutor/env/local/docker-compose.yml -f /home/ubuntu/.local/share/tutor/env/dev/docker-compose.yml -f /home/ubuntu/.local/share/tutor/env/dev/docker-compose.tmp.yml --project-name tutor_dev stop
ERROR: yaml.parser.ParserError: while parsing a block mapping
 in "/home/ubuntu/.local/share/tutor/env/dev/docker-compose.yml", line 66, column 7
expected <block end>, but found '<scalar>'
 in "/home/ubuntu/.local/share/tutor/env/dev/docker-compose.yml", line 66, column 35
Error applying action 'compose:project:started': func=<function _stop_on_local_start at 0x7f78ced225e0> contexts=[]'
Error: Command failed with status 1: docker-compose -f /home/ubuntu/.local/share/tutor/env/local/docker-compose.yml -f /home/ubuntu/.local/share/tutor/env/dev/docker-compose.yml -f /home/ubuntu/.local/share/tutor/env/dev/docker-compose.tmp.yml --project-name tutor_dev stop

Line 66 is: "image: {{ DOCKER_REGISTRY }}overhangio/openedx-authn-dev:{{ MFE_VERSION }}"

Basically the dev image name is not rendered in the template... Thus, we cannot use a function to generate the image name. It's a shame, because it causes some redundancy, but I found no other solution.

[regisb]

@arbrandes FYI I'm going to merge this immediately.