* 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.