cmd/pkg: Build a build context from 'sources'
cmd/pkg: Build a build context from 'sources'

This is the final piece. If 'sources' are defined, tar up
the sources and rewrite them accordingly. Pass it as build
build context to 'docker'.

This allows building from something like this:

  ├── etc
  │   ├── foo
  └── foo
      ├── Dockerfile
          ├── build.yml
          └── main.go

With 'build.yml':

  image: foo
  extra-sources:
    - ../etc:etc

Signed-off-by: Rolf Neugebauer <6fdd1d8677ab4fbcb4df3eff6e190c3298f7f742@docker.com>

furious-luke/linuxkit,JohnnyLeone/linuxkit,YuPengZTE/linuxkit,konstruktoid/linuxkit,yankcrime/linuxkit,konstruktoid/linuxkit,JohnnyLeone/linuxkit,rn/linuxkit,rn/linuxkit,yankcrime/linuxkit,furious-luke/linuxkit,linuxkit/linuxkit,yankcrime/linuxkit,YuPengZTE/linuxkit,djs55/linuxkit,rn/linuxkit,justincormack/linuxkit,djs55/linuxkit,justincormack/linuxkit,YuPengZTE/linuxkit,rn/linuxkit,konstruktoid/linuxkit,YuPengZTE/linuxkit,linuxkit/linuxkit,JohnnyLeone/linuxkit,justincormack/linuxkit,justincormack/linuxkit,yankcrime/linuxkit,djs55/linuxkit,JohnnyLeone/linuxkit,yankcrime/linuxkit,rn/linuxkit,deitch/linuxkit,konstruktoid/linuxkit,furious-luke/linuxkit,djs55/linuxkit,deitch/linuxkit,justincormack/linuxkit,yankcrime/linuxkit,furious-luke/linuxkit,YuPengZTE/linuxkit,deitch/linuxkit,furious-luke/linuxkit,djs55/linuxkit,JohnnyLeone/linuxkit,justincormack/linuxkit,linuxkit/linuxkit,deitch/linuxkit,YuPengZTE/linuxkit,konstruktoid/linuxkit,deitch/linuxkit,konstruktoid/linuxkit,linuxkit/linuxkit,linuxkit/linuxkit