Enable version updates for Docker Compose files

[igor-wl]

Description

Added missing docker-compose config for dependabot updates.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

So that images defined in compose files (i.e. redis:6 is EOL) do not become obsolete without noticing.

[browniebroke]

Could you also update the config for the generated project, please?

https://github.com/cookiecutter/cookiecutter-django/blob/main/%7B%7Bcookiecutter.project_slug%7D%7D%2F.github%2Fdependabot.yml

[igor-wl]

Could you also update the config for the generated project, please?

https://github.com/cookiecutter/cookiecutter-django/blob/main/%7B%7Bcookiecutter.project_slug%7D%7D%2F.github%2Fdependabot.yml

Done