Add first attempt at supporting test suite on ARM

Modify `script/test/default` so it supports a first attempt at testing on ARM.
Call the script with: `$ DOCKERFILE=Dockerfile.armhf script/test/default` to
use the Dockerfile.armhf instead of the default Dockerfile for building the image.

However, running the script is not working fully. The problem is that
`dockerswarm/dind` does not provide an ARM image and therefore cannot be executed.
If that is fixed then we should be able to change the script in order to use the ARM
image instead of the default x86_64 image for running further tests.

Signed-off-by: Jean-Christophe Berthon <[email protected]>

script/test/default

@@ -5,11 +5,15 @@ set -ex
 
 TAG="docker-compose:$(git rev-parse --short HEAD)"
 
+# By default use the Dockerfile, but can be overriden to use an alternative file
+# e.g DOCKERFILE=Dockerfile.armhf script/test/default
+DOCKERFILE="${DOCKERFILE:-Dockerfile}"
+
 rm -rf coverage-html
 # Create the host directory so it's owned by $USER
 mkdir -p coverage-html
 
-docker build -t "$TAG" .
+docker build -f ${DOCKERFILE} -t "$TAG" .
 
 GIT_VOLUME="--volume=$(pwd)/.git:/code/.git"
 . script/test/all