To prevent accidental breakages, test our Docker builds on PRs.
This should be relatively fast thanks to GitHub Actions caching, and the
fact that the NPM `rocksdb` package has a pre-built binary for x64,
which is what `ubuntu-latest` runs.
* perf: shard tests and run them as a matrix in CI
* perf: add concurrency to interrupt prior runs when new ones are pushed
* fix: temporarily lower coverage threshold
* fix: increase propagation delay to account for sporadic test failures
* docs: add comments about concurrency opt
