hadolint: explain ignored rules and point to docs

.hadolint.yaml

 ignored:
+  # For more information, see https://github.com/hadolint/hadolint/wiki/$RULE
+  # Pin versions in apt-get install
   - DL3006
-  - DL3007  
+  # Always tag the version of an image explicitly
+  - DL3007
+  # Using latest is prone to errors if the image will ever update
   - DL3008
+  # Pin versions in pip
   - DL3013