add missing --tag option

Author: CaseyFaist

.travis.yml

@@ -9,10 +9,10 @@ before_script:
   - bundle exec hatchet ci:setup
 
 script:
-  - docker build --pull travis-heroku-16 --file $(pwd)/Dockerfile .
-  - docker run --rm -e "STACK=heroku-16" travis-heroku-16 bash $TESTFOLDER
-  - docker build --pull travis-heroku-18 --file $(pwd)/Dockerfile.heroku-18 .
-  - docker run --rm -e "STACK=heroku-18" travis-heroku-18 bash $TESTFOLDER
+  - docker build --pull --tag heroku-python-build-heroku-16 --file $(pwd)/Dockerfile .
+  - docker run --rm -e "STACK=heroku-16" heroku-python-build-heroku-16 bash $TESTFOLDER
+  - docker build --pull --tag heroku-python-build-heroku-18 --file $(pwd)/Dockerfile.heroku-18 .
+  - docker run --rm -e "STACK=heroku-18" heroku-python-build-heroku-18 bash $TESTFOLDER
 
 jobs:
   include: