Update docker runtime image to Alpine v3.11

[Starbix]

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
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• Pull request includes a sign off
• Code style is correct (run the linters)

Signed-off-by: Cédric Laubacher cedric@laubacher.io

[babolivier]

This all looks good to me. CI is failing on some Sytests run but I'm assuming that's because of matrix-org/sytest#836 (I can't check right now as buildkite is under maintenance but I'd be surprised if it's not that) so I'm going to merge it anyway. Thanks for your contribution! :)