pack: Add '--entry-point'.

pack: Add '--entry-point'.

* guix/scripts/pack.scm (self-contained-tarball): Add #:entry-point and
warn when it's true.
(squashfs-image): Add #:entry-point and honor it.
(docker-image): Add #:entry-point and honor it.
(%options, show-help): Add '--entry-point'.
(guix-pack): Honor '--entry-point' and pass #:entry-point to BUILD-IMAGE.
* gnu/tests/docker.scm (run-docker-test): Test 'docker run' with the
default entry point.
(build-tarball&run-docker-test): Pass #:entry-point to 'docker-image'.
* doc/guix.texi (Invoking guix pack): Document it.
* gnu/tests/singularity.scm (run-singularity-test)["singularity run"]:
New test.
(build-tarball&run-singularity-test): Pass #:entry-point to
'squashfs-image'.