v0.1.1-preview7: Context first: build subcommand

Container first CLI version. Runs build, exec and run in a container.

• Build packs code cleanup: fixes to how it downloads and manages PATH
• Checks consistently for sane environment variables settings in the YAML
• Regression: -exec-prefix flag for build is back and running
• Checks all docker mounts, if it doesn't exist locally, give up
• workspace has subcommands that helps manage auto created $HOME/.yourbase/workspace/ dirs
• Regression: build actually searches and uses build target names defined in the YAML
• Timings for Docker container stand up and tools installations
• Container workdir should be set to a default
• Chore: dep Narwhal v0.5.2