Move addons-frontend image config to seperate docker-compose.yml

[b4handjr]

As seen here, moving the addons-frontend config within the base docker-compose.yml file to the docker-compose.selenium.yml file would fix that issue.

The secondary compose.yml should probably be renamed to docker-compose.ui-tests.yml or something like that.

[EnTeQuAk]

Note that with #5927 there now already is a tests/ui/docker-compose.selenium.yml file.

[b4handjr]

Yup, what do you think about renaming it?

[EnTeQuAk]

I really don't have a preference, we could even drop the .selenium postfix and just name it docker-compose.yml 🤷‍♂️

[b4handjr]

Well I think it should have some kinda reference, but okay good to know.

[willdurand]

See also: #6120

[b4handjr]

@willdurand that was fixed here mozilla/addons-server#10143.