Update rabbitmq
Update rabbitmq

Changes:

- https://github.com/docker-library/rabbitmq/commit/dbbe820: Update to 3.7.15, Erlang/OTP 21.3.8.2, OpenSSL 1.1.1c
- https://github.com/docker-library/rabbitmq/commit/767bbb9: Update to 3.8.0-beta.4, Erlang/OTP 21.3.8.2, OpenSSL 1.1.1c
- https://github.com/docker-library/rabbitmq/commit/e2ddb26: Update the openssl bits of the template to include the full list of all OpenSSL maintainer's key fingerprints (new release is not signed by Matt Caswell)

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