Make optional run osbuilder scripts without docker

[jcvenegas]

osbuilder scripts and makefile targets are configured to using a docker container.

It should be possible to use the scripts/targets without docker

• If the following targets are executed, the image should be created without docker if all the dependencies are installed:

  make rootfs
  make image

• Add a way to osbuilder makefile targets to use docker to build. A variable called USE_DOCKER should tell to makefile to use docker to run osbuilder scripts.

  export USE_DOCKER
  make rootfs
  make image

  The targets above will use docker (docker build/run) to generate an image. Note that the only dependency will be docker.

[jcvenegas]

This is helpful when clear containers is used as default runtime. Because qemu 9pfs issues is not possible to use osbuilder scripts when clear containers is the default docker runtime. see #8