[Feature Request] add "fastblock" feature

[bddap]

I commonly find myself manually changing blocktime before uploading a custom "fastblock" docker image for use in sdk integration tests. It would be nice to have a "fastblock" feature in "Cargo.toml" that would enable to that automatically. Also, a way to automatically build and push a <git-commit-sha>-fastblock docker image.

[bddap]

The docker image thing can probably be accomplished by accepting a new build argument in Dockerfile.