Get image tag from deploy_app task inputs (ONSdigital#32)

ci/README.md

@@ -5,6 +5,7 @@ To deploy this application with Concourse, you must be logged in to a Concourse
 ---
 
 Make sure the required [environment variables](../README.md#environment-variables) are set.
+IMAGE_TAG can be passed as an environment variable or as a file input located at `./image-tag/tag`.
 
 To deploy the app via Concourse, use the following task command:
 

ci/deploy.yaml

@@ -15,6 +15,8 @@ params:
   MAX_INSTANCES:
 inputs:
   - name: eq-questionnaire-launcher
+  - name: image-tag
+    optional: true
 run:
   path: bash
   args:
@@ -26,6 +28,9 @@ run:
       EOL
 
       gcloud auth activate-service-account --key-file $GOOGLE_APPLICATION_CREDENTIALS
-
+      FILE=./image-tag/tag
+      if [[ -f "${FILE}" ]]; then
+        export IMAGE_TAG=`cat "$FILE"`
+      fi
       cd eq-questionnaire-launcher
       ./ci/deploy_app.sh