Publish standalone regtest image

[zone117x]

Follow up to the standalone regtest image implemented in #1476

... new standalone regtest dockerfile which is epoch 2.1 capable. This is intended to be used in CI/local testing as an alternative to the existing standalone docker image (which uses a mocknet stacks-node that is non-functional for epoch 2.1 testing).

This PR adds a workflow_dispatch (manually triggered CI) config to build the new image.

• Multi-platform images are built (for both arm64 and x86).
• This currently uses git commit SHAs for building both the API and stacks-node, so the git tag-based version reporting (/v2/info and /extended/v1/status) will show the commit hashes but the branch/version will be stubbed. This can be fixed once the Stacks 2.1 related branches on the blockchain repo and API repo become more stabilized.
• The docker image tag is currently ${date YYYYMMDDHH}-${STACKS_API_SHA}-${BLOCKCHAIN_SHA} -- this can also be improved later once the repo branches stabilize.
  • The datetime is included to give a human readable idea of a "version", until actual version tags are stable and ready to use.
• Several techniques are used to reduce docker build time, which otherwise would take several hours for multi-platform stacks-node and API building:
  • The stacks-node binary is cross-compiled on the runner host and then copied into the docker image during build.
  • Cargo's new multi-target build feature is used.
  • A rust-specific github caching action is used.
  • Docker-registry caching is enabled.
• Despite the above speedups, a "cold build" (no cache hits) can take ~1 hour, so later on, this workflow should probably only be enabled on merge to a release branch.

Currently, this workflow must be ran with this blockchain commit: stacks-network/stacks-core@b16e121

See example of running this workflow: https://github.com/hirosystems/stacks-blockchain-api/actions/runs/3648003869/jobs/6161211248

And the image it published: https://hub.docker.com/layers/hirosystems/stacks-blockchain-api-standalone-regtest/2022120812-88f446b-b16e121/images/sha256-726d8b52bdea017ed3f46f1341da755f288b4a94caace5a017a2799a56914bc0?context=explore

[CharlieC3]

LGTM!

[CharlieC3]

Is this intended to be disabled?

[zone117x]

Yep, I think latest implies more stability than we have right now, probably can enable once the next couple PRs that this depends on are merged.