hardcoding docker image versions

.gitlab-ci.yml

@@ -18,7 +18,7 @@ stages:
 
 build:sdist:
   stage: build_sdist
-  image: 3.5.5-alpine3.7
+  image: python:3.5.5-alpine3.7
   script:
       - python3 setup.py sdist
   artifacts:
@@ -71,7 +71,7 @@ test:
 
 release:pypi:
     stage: release
-    image: 3.5.5-alpine3.7
+    image: python:3.5.5-alpine3.7
     script:
         - pip3 install twine
         - twine upload dist/$SDIST -u $PYPI_USER -p $PYPI_PASS