docker-compose.yml SETTINGS_ env var family typo

[dearlordylord]

SETTINGS_ prefixed env vars don't end up in settings

I believe you meant SETTING_ : those properly end up in settings

e.g. bucket names ain't empty when I run docker exec -u zulip -it docker-zulip-zulip-1 bash -l -c "/home/zulip/deployments/current/manage.py transfer_uploads_to_s3"

Fixes: ?

How did you test this PR?

transfer_uploads_to_s3 script works now

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