Allow installation of additional modules through environment variables

[PaulLebmann]

Hi,

since I'm using boost on a D7 site, I recently realized, that during the migration to hostmaster on docker I lost my additional drush modules.

Since downloading modules in the form of drush dl --destination=/var/aegir/.drush/commands provision_boost module_2 ... works I'd like to propose the idea that one could use two environment variables $DRUSH_ADDITIONAL_MODULES and $HOSTMASTER_ADDITIONAL_MODULES which could be checked in docker-entrypoint.sh and accordingly downloaded after the download of provision and the installation section of the hostmaster.

Do you think this is possible / useful?

best regards