pack: Add '--format' option and Docker output support.

* guix/docker.scm: Remove dependency on (guix store) and (guix utils).
Use (guix build store-copy).  Load (json) lazily.
(build-docker-image): Remove #:system.  Add #:closure, #:compressor, and
'image' parameters.  Use 'uname' to determine the architecture.  Remove
use of 'call-with-temporary-directory'.  Use 'read-reference-graph' to
compute ITEMS.  Honor #:compressor.
* guix/scripts/pack.scm (docker-image): New procedure.
(%default-options): Add 'format'.
(%formats): New variable.
(%options, show-help): Add '--format'.
(guix-pack): Honor '--format'.
* guix/scripts/archive.scm: Remove '--format' option.  This reverts
commits 1545a012,
01445711, and
03476a23.
* doc/guix.texi (Invoking guix pack): Document '--format'.
(Invoking guix archive): Remove documentation of '--format'.