Replace calls to docker composer by API calls

[gdethier]

Currently, docker composer is called via an explicit process execution. Docker composer should be replaced by calls to Docker API. Indeed, the user of composer is simple enough to be replaced by a manual setup via the API.

[gdethier]

Before closing this issue, data volumes management should be addressed (allow them to remain upon clear action but add option to delete them later and/or at the same time).

[gdethier]

A volume has been added to keep WP files in addition to DB data. Volumes are not cleared automatically so a start/clear/start sequence does not loose data.