pack: Docker backend now honors '--localstatedir'.

* guix/docker.scm (build-docker-image): Add #:database parameter.
Create /var/guix/db, /var/guix/profiles, etc. when DATABASE is true.
* guix/scripts/pack.scm (docker-image): Export.  Remove #:deduplicate?
parameter.  Define 'database' and pass it to 'docker-image'.
* tests/pack.scm (test-assertm): Recompile the derivation of
%BOOTSTRAP-GUILE.
("docker-image + localstatedir"): New test.

guix/docker.scm

@@ -26,6 +26,7 @@
                           delete-file-recursively
                           with-directory-excursion
                           invoke))
+  #:use-module (gnu build install)
   #:use-module (json)                             ;guile-json
   #:use-module (srfi srfi-19)
   #:use-module (srfi srfi-26)
@@ -108,11 +109,15 @@ return \"a\"."
                              (symlinks '())
                              (transformations '())
                              (system (utsname:machine (uname)))
+                             database
                              compressor
                              (creation-time (current-time time-utc)))
   "Write to IMAGE a Docker image archive containing the given PATHS.  PREFIX
 must be a store path that is a prefix of any store paths in PATHS.
 
+When DATABASE is true, copy it to /var/guix/db in the image and create
+/var/guix/gcroots and friends.
+
 SYMLINKS must be a list of (SOURCE -> TARGET) tuples describing symlinks to be
 created in the image, where each TARGET is relative to PREFIX.
 TRANSFORMATIONS must be a list of (OLD -> NEW) tuples describing how to
@@ -188,10 +193,15 @@ SRFI-19 time-utc object, as the creation time in metadata."
                                 source))))
                   symlinks)
 
+        (when database
+          ;; Initialize /var/guix, assuming PREFIX points to a profile.
+          (install-database-and-gc-roots "." database prefix))
+
         (apply invoke "tar" "-cf" "layer.tar"
                `(,@transformation-options
                  ,@%tar-determinism-options
                  ,@paths
+                 ,@(if database '("var") '())
                  ,@(map symlink-source symlinks)))
         ;; It is possible for "/" to show up in the archive, especially when
         ;; applying transformations.  For example, the transformation
@@ -203,7 +213,11 @@ SRFI-19 time-utc object, as the creation time in metadata."
         (system* "tar" "--delete" "/" "-f" "layer.tar")
         (for-each delete-file-recursively
                   (map (compose topmost-component symlink-source)
-                       symlinks)))
+                       symlinks))
+
+        ;; Delete /var/guix.
+        (when database
+          (delete-file-recursively "var")))
 
       (with-output-to-file "config.json"
         (lambda ()