Changed pycodestyle job.

.gitlab-ci.yml

-image: docker:stable
-
-before_script:
-  - apk add --no-cache py-pip
-  - pip install pycodestyle
-
 pycodestyle:
+  image: docker:stable
+  tags:
+    - docker
+  before_script:
+    - apk add --no-cache py-pip
+    - pip install pycodestyle
   script:
     # Run pycodestyle (PEP8) check.
     # https://pycodestyle.readthedocs.io/en/latest/intro.html#error-codes