Push multi-arch docker images

[ddelange]

👋 I did some awesome work for the Pelias project and would love for everyone to have a look at it and provide feedback.

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Here's the reason for this change 🚀

Towards pelias/docker#264

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Here's what actually got changed 👏

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Here's how others can test the changes 👀

[ddelange]

closing:

• builds are not being triggered, because this PR is coming from a fork. probably need a maintainer to set this up ref ARM support docker#264 (comment)
• unclear in which order to roll this out, and will probably need a maintainer to time it/roll it out ref ARM support docker#264 (comment) and onwards