Use global job for building docker image

Change-Id: I4a2a3b28ed2aa7f227706153ac94f1de25e2609a
Story: 2001694
Task: 23213

.zuul.yaml

@@ -85,6 +85,7 @@
       jobs:
         - monascalog-python2-tempest
         - monascalog-python3-tempest
+        - docker-build-monasca-service
     gate:
       queue: monasca
       jobs:

docker/build_image.sh

@@ -44,6 +44,10 @@ set -eo pipefail  # Exit the script if any statement returns error.
 # to provide it as in the following example:
 #   $ ./build_image.sh master master refs/changes/19/595719/3
 
+# Go to folder with Docker files.
+REAL_PATH=$(python -c "import os,sys; print(os.path.realpath('$0'))")
+cd "$(dirname "$REAL_PATH")/../docker/"
+
 [ -z "$DOCKER_IMAGE" ] && \
     DOCKER_IMAGE=$(\grep DOCKER_IMAGE Dockerfile | cut -f2 -d"=")