lib/begin: added support for beginning from remote docker images
lib/begin: added support for beginning from remote docker images

This commit pulls in docker2aci to add support for commands such as:

    acbuild begin docker://alpine

Beginning from local docker images remains unimplemented, and will be
added in a future commit.

dgonyeo/acbuild,dgonyeo/acbuild,appc/acbuild,containers/build,appc/acbuild,jonboulle/acbuild,joshix/acbuild,jonboulle/acbuild,joshix/acbuild,containers/build