`composer server stop` doesn't stop mutagen container

[mikelittle]

The command composer server stop doesn't stop the mutagen container, regardless of whether you add the --mutagen flag.

Steps to reproduce:

1. Start a local server stack with composer server start --mutagen
2. Stop the stack with composer server stop --mutagen
3. Run docker ps. Observe that both the mutagen and altis-proxy container are still running. The latter is expected, but the former is not.

I expect the mutagen container to be stopped along with the others leaving only the altis-proxy container.

Acceptance criteria:

• Typing composer server stop should stop the mutagen container if it was started.

[mikelittle]

I'm tempted to close this, but technically, we still support mutagen.