bugfix: changed externals output arcive diretory permissions to docke…

[akoerner1]

In order to save bandwidth and docker API calls and under normal circumstances the libadore/externals libraries/submodules are not populated or built. On first build of libadore each externals build context is pulled from docker and saved to /var/tmp. On subsequent builds the docker context is loaded from /var/tmp/docker to prevent repeated docker api calls.

This PR fixes a bug where one user pulls the images and saves them to /var/tmp/docker and another user tries to access them.
The directory /var/tmp/docker is created on first-ever build of libadore on the system and the permissions are set to the group docker. Any member of the docker group can then subsequently read and write to /var/tmp/docker

This is documented in the readme as part of the PR, as well as, an explicit make target was added to advertise this cache and provide a means to delete/clean it.

make help
Usage: make <target>
clean_external_library_cache             Clean/delete libadore system wide cache in /var/tmp/docker. Note: this is never done automatically and must be manually invoked.