automation: Update docker-mastodon Docker tag to v4.3.8

This MR contains the following updates:

Package	Update	Change
docker-mastodon	patch	v4.3.7 -> v4.3.8

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This MR has been generated by Renovate Bot.

enabled an automatic merge when the pipeline for bf586a0b succeeds

mentioned in issue #2

merged

mentioned in commit 1066aa86