builder/docker: test StepTempDir
builder/docker: StepPull test, driver abstraction for tests
builder/docker: StepRun tests
builder/docker: StepExport tests
builder/docker: artifact
website: start documenting the builder
website: document dockerfile
builder/docker: config refactor
builder/docker: ability to disable pull
website: doc pull
builder/docker: show stderr if docker fails to run
builder/docker: UploadDir
provisioner/chef-solo: Say using Chef-solo
builder/docker: stream output from commands
Merge branch 'f-docker-builder': Docker builder

This introduces a Docker builder. The docker builder is able to create
containers by starting an existing Docker image, provisioning it using
standard practices, and then exporting it using `docker export`.
Update CHANGELOG
builder/docker: remove unused file
builder/docker: say when killing the container
website: better document the dockerfile interaction with the builder
builder/docker: verify docker is available on path