tests/docker/Makefile.include: add a generic docker-run target

This re-factors the docker makefile to include a docker-run target which
can be controlled entirely from environment variables specified on the
make command line. This allows us to run against any given docker image
we may have in our repository, for example:

    make docker-run TEST="test-quick" IMAGE="debian:arm64" \
         EXECUTABLE=./aarch64-linux-user/qemu-aarch64

The existing docker-foo@bar targets still work but the inline
verification has been dropped because we already don't hit that due to
other pattern rules in rules.mak.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

Message-Id: <20161011161625.9070-5-alex.bennee@linaro.org>
Message-Id: <20161011161625.9070-6-alex.bennee@linaro.org>
[Squash in the verification removal patch. - Fam]
Signed-off-by: Fam Zheng <famz@redhat.com>
