Remove volumes on docker run -rm

If we don't care about the container then we don't care about any
volumes created with the run of that container

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Bump to version v0.7.5

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Change version to v0.7.5

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Builder: extract the execution of a single build step into buildFile.BuildStep

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
New build instruction: ONBUILD defines a trigger to execute when extending an image with a new build

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)