[Improvement]: mailpit container_name docker-compose.yaml

[derhecht]

Improvement description

Every container which is starting up by docker compose command will generate a custom name based on folder/project name. Expect the mailpit container.
This will conflict e.g. if you running multiple instances of pimcore on one machine or have an already standalone instance of mailpit running.

Improvement: Remove container_name directive from docker-compose.yaml file.

The mailpit service is referenced by its service name which is mailpit - for every setup - not by its container name.

[blankse]

@derhecht I created a PR: #589

[alexz707]

Thanks for bringing that up and thanks for the PR, already merged