* guix/docker.scm (build-docker-image): Rename "path" argument to
"prefix" to reflect the fact that it is used as a prefix for the
symlink targets. Add the "paths" argument, and remove the "closure"
argument, since it is now redundant. Add a "transformations"
argument.
* guix/scripts/pack.scm (docker-image): Read the profile's reference
graph and provide its paths to build-docker-image via the new "paths"
argument.
* guix/scripts/pack.scm (self-contained-tarball): Add #:target.
(docker-image): Add #:target.
[build]: Pass it to 'build-docker-image'.
(%options, show-help): Add '--target'.
(guix-pack): Pass TARGET to 'profile-derivation' and to 'build-image'.
* guix/docker.scm (build-docker-image): Add #:system parameter and honor it.
* doc/guix.texi (Invoking guix pack): Document '--target'.
(Additional Build Options): Refer to the Autoconf manual instead of the
obsolete 'configure.info' for cross-compilation.
* guix/docker.scm: New file.
* Makefile.am (MODULES): Register it.
* guix/scripts/archive.scm (show-help, %options, guix-archive): Add
support for "--format".
* doc/guix.texi (Invoking guix archive): Document it.