[FLINK-21071][release][docker] Update docker dev branch in snapshot branch

[zentol]

Updates the release scripts to automatically update the flink-docker dev-* branch reference when creating the release-* branch.

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Last check on commit 29556a8 (Fri May 28 07:13:45 UTC 2021)

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• No documentation files were touched! Remember to keep the Flink docs up to date!

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[rmetzger]

Thx for extending this script! +1 to merge