add info and example of image cache in use when building

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)

Docker-DCO-1.0-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #3544 from SvenDowideit/build-image-cache

add info and example of image cache in use when building
Port 'docker images' to the engine API

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Add compat 1.8

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
add legacy compat

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
move legacy stuff outside the job

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
Merge pull request #3575 from shykes/engine-get-images

move `docker images` to a job
Fix docs Dockerfile to install correctly again

Previously we've been getting:

```
Step 6 : RUN pip install Sphinx==1.1.3
 ---> Running in 397eab32f57a
 Wheel installs require setuptools >= 0.8 for dist-info support.
 pip's wheel support requires setuptools >= 0.8 for dist-info support.
 Storing debug log for failure in /.pip/pip.log
 2014/01/13 18:01:34 The command [/bin/sh -c pip install Sphinx==1.1.3] returned a non-zero code: 1
 make: *** [docs] Error 1
```

This fixes that by telling pip not to use sudo to install (since we're already root).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3584 from tianon/fix-docs-dockerfile

Fix docs Dockerfile to install correctly again
Fix docs typo: "Gento" -> "Gentoo"

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3587 from tianon/fix-gentoo-typo

Fix docs typo: "Gento" -> "Gentoo"