Add cache for getting docker registry authenticated session (#328)
Add cache for getting docker registry authenticated session (#328)

It is not strictly correct as the authenticated session has an
expiration date (on the official docker hub it is valid for 600s).
In such case the `TokenExpiredError` is raised.

We assume it won't happen: dmake execution should be much faster than
that. If it breaks in practice, we will have to implement our own
cache which checks the expiration date.

Test on dmake itself:
- before: 3.953s
- after:  3.088s
Deepomatic/dmake,Deepomatic/dmake,Deepomatic/dmake,Deepomatic/dmake