ci: build on-demand docker images for testing on devnets

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This PR:

• Implements a new Dockerfile designed to quickly build and release a multi-arch docker image from any branch based on a semver tag provided by developers, as requested by @PastaPastaPasta here. Example built images are here. Note that these are not yet gitian builds, but I could modify this to do gitian builds instead (probably with a performance impact?)
• Implements CI that integrates with this new AWS infrastructure repository, allowing builds to execute quickly on powerful ephemeral self-hosted runners. Build time is currently around 25 minutes.
• Modifies existing CI to use an improved caching mechanism, and renames the existing release Dockerfile to live alongside the new Dockerfile for building alpha releases

[PastaPastaPasta]

Please resolve conflicts

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I don't know how to do this. I had to develop on master because GitHub applies actions from the default branch. But since I cannot do a PR into master in the upstream repo, and the directory structure has changed, we now have this conflict. How do I resolve this?

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Closed in favour of #4667