pack: Allow embedding custom control files in deb packs.

* guix/scripts/pack.scm (self-contained-tarball/builder)
[extra-options]: New argument.
(self-contained-tarball, squashfs-image, docker-image)
(debian-archive): Likewise.  Remove two TODO comments.  Document
EXTRA-OPTIONS.  Use the custom control files when provided.
(%deb-format-options): New variable.
(show-deb-format-options, show-deb-format-options/detailed): New procedures.
(%options): Register new options.
(show-help): Augment with new usage.
(guix-pack): Validate and propagate new argument values.
* doc/guix.texi (Invoking guix pack)[deb]: Document how to list advanced
options.  Add an example.
* tests/pack.scm (deb archive...): Provide extra-options to the debian-archive
procedure, and validate that the provided files are embedded in the pack.

1 files changed, 8 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index cca46218f2..b3c16e6507 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -6047,6 +6047,14 @@ such file or directory'' message.
 This produces a Debian archive (a package with the @samp{.deb} file
 extension) containing all the specified binaries and symbolic links,
 that can be installed on top of any dpkg-based GNU(/Linux) distribution.
+Advanced options can be revealed via the @option{--help-deb-format}
+option.  They allow embedding control files for more fine-grained
+control, such as activating specific triggers or providing a maintainer
+configure script to run arbitrary setup code upon installation.
+
+@example
+guix pack -f deb -C xz -S /usr/bin/hello=bin/hello hello
+@end example
 
 @quotation Note
 Because archives produced with @command{guix pack} contain a collection