ci: release flow with docker and binary artifacts

[rafa-stacks]

Everything needed to cut a stacksup release: a version-bump script, a dispatch-driven release workflow, and the Dockerfile for a published image.

./release.sh 0.2.0            # bumps Cargo.toml [package] version + syncs Cargo.lock
# PR + merge the bump, then tag the merged commit:
git tag v0.2.0 && git push origin v0.2.0
# dispatch the Release workflow with version 0.2.0