Description of problem:
When creating a repository of content_type docker, it is not possible to define download_policy to "on_demand", as it is the case and correctly documented for the content_type yum:

[root@satellite ~]# hammer repository update --help |grep -A1 download-policy
 --download-policy DOWNLOAD_POLICY             Download policy for yum repos (either 'immediate',
                                               'on_demand', or 'background (deprecated)')

Example on one single repository, in which the initial sync downloaded all blobs:

[root@satellite ~]# du -sk /var/lib/pulp ; hammer repository sync --id 16 --organization-id 1 ; du -sk /var/lib/pulp
3147168	/var/lib/pulp
[....................................................................................................................] [100%]
New blobs: 924.
9165392	/var/lib/pulp
[root@satellite ~]#

That's roughly 5.7GB used by one single container, out of 117 different ones required by RHOSP 16.1. The downloaded tags include not only older versions that will never be used, but also others with tags such as *-source.

Being able to download images as they are pulled would save a lot of space (> 100GB on the initial sync of RHOSP 16.1) and also make the initial sync way faster. Only when an image is being pulled from satellite, the required blobs would be downloaded, depending on the tag specified as part of the pull.

Version-Release number of selected component (if applicable):
Satellite 6.9.5 or earlier

How reproducible:
Reproducible via web or hammer cli

Steps to Reproduce:
1. Create a product
[root@satellite ~]# hammer product create --organization-id 1 --name "OSP16.1 Containers"
Product created.
[root@satellite ~]# 

2. Attempt to create repository using donwload-policy:
[root@satellite ~]# hammer repository create \
>   --name base \
>   --organization-id 1 \
>   --product "OSP16.1 Containers" \
>   --content-type docker \
>   --url https://registry.redhat.io \
>   --docker-upstream-name rhosp-rhel8/openstack-base \
>   --upstream-username $USERNAME \
>   --upstream-password $PASSWORD \
>   --download-policy on_demand
Could not create the repository:
  Validation failed: Download policy Cannot set attribute download_policy for content type docker
[root@satellite ~]# 


Actual results:
Can not use download_policy "on_demand" for content type "docker"

Expected results:
Can use download_policy "on_demand" for content type "docker"