release_config: container is a repeated string field

Container is a string, and a flag can be in more than one container.

Bug: 328495189
Test: manual
Ignore-AOSP-First: cherry-pick
Merged-In: I5a2a9855532027584d2b67f63f1b9584fce3d8d9
Change-Id: I5a2a9855532027584d2b67f63f1b9584fce3d8d9