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)
Update mkimage-arch.sh

mktemp creates a root directory ("/") with permissions set to 700. Default should be 755 so other users in the container can access its subdirs (e.g http user for nginx for /srv/http/test/index.html).

Docker-DCO-1.1-Signed-off-by: Samuel Andaya <samuel@andaya.net> (github: sandaya)
Merge pull request #3897 from sandaya/master

Update mkimage-arch.sh to set proper permissions on rootfs directory
missing quote quota

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
i was reading, and it sprang out at me :)

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #3920 from SvenDowideit/hack-spelling

Fix hack/MAINTAINERS.md spelling problems
improve the Vagrant VMs

This bumps the VM memory and corrects forwarded ports when they're used
already.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
iptables: use dest_addr and dest_port for public port FORWARD rule

Docker-DCO-1.1-Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> (github: jpoimboe)
Merge pull request #3919 from SvenDowideit/missing-quote

missing quote quota
Merge pull request #3922 from jpoimboe/iptables-public-dest-port-fix

iptables: use dest_addr and dest_port for public port FORWARD rule
Merge pull request #3557 from unclejack/improve_vagrantfile

improve the Vagrant VMs: bump RAM to 512 MB & correct forwarded ports
builder: clearly display ONBUILD triggers during a build.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #3254 from shykes/onbuild

New build instruction: ONBUILD defines a trigger to execute when extending an image with a new build
docs: Installation on Mac OS X 10.6 Snow Leopard
Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #3918 from ostezer/master

docs: Requirements And Installation on Mac OS X 10.6 Snow Leopard
Update .mailmap and AUTHORS

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3926 from tianon/update-mailmap

Update .mailmap and AUTHORS
Bump to version 0.8.0
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)