We used to create dockerfiles blindly assuming there is content, but
that's not the case for virtual packages.
Due to https://github.com/moby/moby/issues/38039 we are forced for a
"unpleasant" workaround, as we can't create empty FROM scratch images
and export them.
Drop image-repository on create-repo. In case of a docker repository, --output is the image reference to use.
Also restore default output build dir.
See also: #169
* installer.LuetRepository is now installer.LuetSystemRepository,
a struct that extend config.LuetRepository
* config: system_repositories option is now "repositories".
This resolve viper issue.
* config: cache_repositories option is now "repetitors"
This resolve viper issue.
* cmd/*: Now use new config.LuetRepository
* cmd/search: now create local luet repository if database_engine
is equal to "boltdb"
