Container Support

Should this story be for "mirror" and "additive" sync modes?

Mirror-on-sync would be a very welcome addition to the base user story.

https://pulp.plan.io/issues/2602

In order to implement the auth mechanism required for docker, the following objects and methods need to be added.

DockerDownloader needs to inherit from HttpDownloader[0]. The downloader's init needs to accept a keyword argument called 'auth' which is a dictionary. The downloader needs to store the auth dict as a property of the downloader - to be used by run().

The run method of the DockerDownloader needs to add the information from the auth property to the session. Then it needs to perform the download like here[1]

The DockerRemote needs to override download_factory[2] method. The DownloadFactory needs to be instantiated with downloader_overrides that contain a dict with {'https': DockerDownloader}.

The get_downloader() method of DockerRemote needs to check if auth has already been performed against the remote source. If not, perform auth. Then pass the 'auth' dict to the build() method of the factory[3].

[0] https://docs.pulpproject.org/en/3.0/nightly/plugins/plugin-api/download.html#httpdownloader
[1] https://github.com/pulp/pulp/blob/master/plugin/pulpcore/plugin/download/http.py#L169
[2] https://docs.pulpproject.org/en/3.0/nightly/plugins/plugin-api/models.html#pulpcore.plugin.models.Remote.download_factory
[3] https://github.com/pulp/pulp/blob/master/plugin/pulpcore/plugin/models/remote.py#L61

Since token auth sometimes requires frequently updating the Authorization header, it would be much simpler if it was possible to update that header for the entire session. I've opened an upstream issue https://github.com/aio-libs/aiohttp/issues/3299