ci: add label to docker images

This allows us or the user to perform batch operations on all
images produced by the ci, e.g. to prune all dangling images,
without affecting non-ci images.

ci/test/02_run_container.sh

@@ -5,6 +5,7 @@
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 export LC_ALL=C.UTF-8
+export CI_IMAGE_LABEL="bitcoin-ci-test"
 
 set -ex
 
@@ -19,6 +20,7 @@ if [ -z "$DANGER_RUN_CI_ON_HOST" ]; then
       --file "${BASE_READ_ONLY_DIR}/ci/test_imagefile" \
       --build-arg "CI_IMAGE_NAME_TAG=${CI_IMAGE_NAME_TAG}" \
       --build-arg "FILE_ENV=${FILE_ENV}" \
+      --label="${CI_IMAGE_LABEL}" \
       --tag="${CONTAINER_NAME}" \
       "${BASE_READ_ONLY_DIR}"
   docker volume create "${CONTAINER_NAME}_ccache" || true