Feature: The generated containers should by default share a composer cache to speed up installation

[mleutenegger]

The host container (and the init command) should share a composer cache. This is relatively simple, as we would simply have to add a named volume to docker.

The following has to be fulfilled before a PR can be merged:

• A named volume is mounted in the default composer cache directory for host containers and in the init docker run command
• The location of the volume inside the container can be customized using a config option
• The volume can be cleared using a command (like ssdev clear-composer-cache)