* fix: allow configuring `docker_repository` for cloud-function2
When docker repository is not specified by default docker repository
`projects/PROJECT_ID/locations/REGION/repositories/gcf-artifacts` is used.
In such a case, terraform plan always generates a difference for `docker_repository`
field as the module passes null value but the tfstate file has the above specified
default value. This fix allows one to prevent unnecessary infrastructure change when
using the default repository as well as any user created repository.
* doc: updated README for cloud-function-v2
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Co-authored-by: Ludovico Magnocavallo <ludomagno@google.com>