Makefile: Rules for docker testing

This adds a group of make targets to run docker tests, all are available
in source tree without running ./configure.

The usage is shown with "make docker".

Besides the fixed ones, dynamic targets for building each image and
running each test in each image are generated automatically by make,
scanning $(SRC_PATH)/tests/docker/ files with specific patterns.

Alternative to manually list particular targets (docker-TEST@IMAGE)
set, you can control which tests/images to run by filtering variables,
TESTS= and IMAGES=, which are expressed in Makefile pattern syntax,
"foo% %bar ...". For example:

    $ make docker-test IMAGES="ubuntu fedora"

Unfortunately, it's impossible to propagate "-j $JOBS" into make in
containers, however since each combination is made a first class target
in the top Makefile, "make -j$N docker-test" still parallels the tests
coarsely.

Still, $J is made a magic variable to let all make invocations in
containers to use -j$J.

Instead of providing a live version of the source tree to the docker
container we snapshot it with git-archive. This ensures the tree is in a
pristine state for whatever operations the container is going to run on
them.

Uncommitted changes known to files known by the git index will be
included in the snapshot if there are any.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1464755128-32490-5-git-send-email-famz@redhat.com