docker: 'build-docker-image' accepts an optional #:entry-point.

* guix/docker.scm (config): Add #:entry-point and honor it.
(build-docker-image): Likewise.

guix/docker.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017 Ricardo Wurmus <rekado@elephly.net>
-;;; Copyright © 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2017, 2018, 2019 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2018 Chris Marusich <cmmarusich@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -73,7 +73,7 @@
   `((,(generate-tag path) . ((latest . ,id)))))
 
 ;; See https://github.com/opencontainers/image-spec/blob/master/config.md
-(define (config layer time arch)
+(define* (config layer time arch #:key entry-point)
   "Generate a minimal image configuration for the given LAYER file."
   ;; "architecture" must be values matching "platform.arch" in the
   ;; runtime-spec at
@@ -81,7 +81,9 @@
   `((architecture . ,arch)
     (comment . "Generated by GNU Guix")
     (created . ,time)
-    (config . #nil)
+    (config . ,(if entry-point
+                   `((entrypoint . ,entry-point))
+                   #nil))
     (container_config . #nil)
     (os . "linux")
     (rootfs . ((type . "layers")
@@ -110,6 +112,7 @@ return \"a\"."
                              (transformations '())
                              (system (utsname:machine (uname)))
                              database
+                             entry-point
                              compressor
                              (creation-time (current-time time-utc)))
   "Write to IMAGE a Docker image archive containing the given PATHS.  PREFIX
@@ -118,6 +121,9 @@ 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.
 
+When ENTRY-POINT is true, it must be a list of strings; it is stored as the
+entry point in the Docker image JSON structure.
+
 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
@@ -227,7 +233,8 @@ SRFI-19 time-utc object, as the creation time in metadata."
       (with-output-to-file "config.json"
         (lambda ()
           (scm->json (config (string-append id "/layer.tar")
-                             time arch))))
+                             time arch
+                             #:entry-point entry-point))))
       (with-output-to-file "manifest.json"
         (lambda ()
           (scm->json (manifest prefix id))))