Mailhog container missing when using .magento.docker.yml

Nobody has claimed this yet.

Behavior of Mailhog differs when using different configuration methods. When using the `.magento` directory for configuration it's not necessary to explicitly define Mailhog. Mailhog will be included by default. Confusingly, when using the `.magento.docker.yml` for configuration, it's necessary to define a Mailhog container.

This also violates the `build:compose` command interface, which contains a `--no-mailhog` option. A Mailhog container should be generated by default in all situations, unless the `--no-mailhog` option is utilized.

### Preconditions
Using a .magento.docker.yml configuration file without a Mailhog definition

### Steps to reproduce
Run `build:compose` close to default. Eg. `vendor/bin/ece-docker build:compose --mode=developer --sync-engine=native --with-xdebug`

### Expected result
The generated Docker configuration would include a Mailhog container.

### Actual result
The generated Docker configuration dosen't include a Mailhog container.

Start at the build:compose command and compare how .magento configuration and .magento.docker.yml are processed, using the reproduction command in the issue. Verify the generated Docker configuration for a Mailhog container with no explicit definition, and confirm that --no-mailhog still omits it.

Written by the indexing model from the issue text.