Adds required Azure backup env variable WALG_AZ_PREFIX to documentation

[bloever]

Current documentation in the postgres-operator fails to mention that WALG_AZ_PREFIX enables Azure Backups. This is a simple fix for the documentation so others won't be stranded.

Spilo image env variables: (https://github.com/zalando/spilo/blob/master/ENVIRONMENT.rst)

[FxKu]

WALG_AZ_PREFIX is generated by spilo if wal_az_storage_account is configured. I think, that's why @apeschel left this variable out in #1537. You can overwrite it, but not sure if everything still works then.

[bloever]

My experience was that there was nothing in /run/etc/wal-e.d/env before i manually configured WALG_AZ_PREFIX. wal_az_storage_account was set from the start.

I guess this is the relevant part in the Spilo image?
(https://github.com/zalando/spilo/blob/b2b1c6988c81ed4705c08b1902712251b86a4f75/postgres-appliance/scripts/configure_spilo.py#L819)

[FxKu]

Ok, just learned that I mixed this variable up with the scope prefix I guess. So it might indeed be needed.

[FxKu]

👍

[Jan-M]

👍

[FxKu]

Thanks again @bloever