🚧 Travis: Fix build stage

.travis.yml

@@ -23,7 +23,7 @@ jobs:
 
         # We are using the images to cache, so pull and tag it
         - |
-          for x in sub_images; do
+          for x in $sub_images; do
             docker pull $DOCKER_USERNAME/my_ocrd_workflow-$x || true
             docker tag $DOCKER_USERNAME/my_ocrd_workflow-$x my_ocrd_workflow-$x || true
           done
@@ -32,13 +32,13 @@ jobs:
 
         - |
           set -e
-          for x in sub_images; do
+          for x in $sub_images; do
             docker tag my_ocrd_workflow-$x $DOCKER_USERNAME/my_ocrd_workflow-$x:$TRAVIS_COMMIT
           done
         - docker images
         - |
           set -e
-          for x in sub_images; do
+          for x in $sub_images; do
             docker push $DOCKER_USERNAME/my_ocrd_workflow-$x:$TRAVIS_COMMIT
           done