support building with host packaging tools

[mcandre]

Allow ROCKHOPPER_IMAGE to be blank.

When blank, look for rocklet in standard host path.

Emit a log message (warning? info?) that we are using host tools.

Bypass docker run... and execute the rocklet directly.

Document that rocklet executables, often bundled into Docker images, would then be the fundamental unit.

Rewards:

• More ways to debug package builds.
• Support more host environments, where Docker may not be an option.
• Lightweight.

Depends on #38