Support multiple container repos on manual cleanup

What does this MR do?

The code snippet currently does not support container registries containing multiple ContainerRepository objects and it will only grab the first one.

It is necessary to modify the code so we can loop through each ContainerRepository object, and then run the cleanup.

The example snippets below show a project containing two ContainerRepository objects.

Rails output before

irb(main):220:0> repo = ContainerRepository.find_by(project_id: P)
=> #<ContainerRepository id: 15, project_id: 336, name: "first-repo", created_at: "2020-09-09 22:06:30", updated_at: "2020-09-09 22:06:30", status: nil>
irb(main):224:0> Projects::ContainerRepository::CleanupTagsService.new(project, user, policy.attributes.except("created_at", "updated_at")).execute(repo)
=> {:deleted=>[], :status=>:success}

Rails output after

irb(main):184:0> project.container_repositories.find_each do |repo|
irb(main):185:1*
irb(main):186:1*     puts repo.attributes
irb(main):187:1>
irb(main):188:1>     # Get the expiration policy
irb(main):189:1>     policy  = ContainerExpirationPolicy.find_by(project_id: P)
irb(main):190:1>
irb(main):191:1>     # Start the tag cleanup
irb(main):192:1>     puts Projects::ContainerRepository::CleanupTagsService.new(project, user, policy.attributes.except("created_at", "updated_at")).execute(repo)
irb(main):193:1>
irb(main):194:1> end
{"id"=>15, "project_id"=>336, "name"=>"first-repo", "created_at"=>Wed, 09 Sep 2020 22:06:30 UTC +00:00, "updated_at"=>Wed, 09 Sep 2020 22:06:30 UTC +00:00, "status"=>nil}
{:deleted=>[], :status=>:success}
{"id"=>16, "project_id"=>336, "name"=>"second-repo", "created_at"=>Wed, 09 Sep 2020 22:09:07 UTC +00:00, "updated_at"=>Wed, 09 Sep 2020 22:09:07 UTC +00:00, "status"=>nil}
{:deleted=>[], :status=>:success}
=> nil
irb(main):195:0>

Relates to #219915 (closed)

Another user also pointed out this issue here #219915 (comment 398622303)

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