Remove LABEL version from Dockerfile and CONTRIBUTING guide

Since this is at the top of the Dockerfile this avoid us to share
layers between different docker images even if the image share most of it.

Author: humitos

CONTRIBUTING.rst

@@ -46,20 +46,17 @@ are defined in the :doc:`README`.
 
 We follow `semantic versioning`_, but drop the bug fix level version number for
 our images, as this level of granularity is not important for any application of
-these images. Each release is tagged in Git, and the release version number is
-included in the Dockerfile version label (``LABEL version="2.0"``).
+these images.
 
 Releases should be merged into one of the ``releases/`` branches, for instance
-``releases/2.x``. The version label in the Dockerfile should be updated to the
-next version in the series, following semver rules. This commit should then also
-be tagged using the new version number.
+``releases/2.x``. This commit should then also be tagged using the new version number.
 
-If the version number in the Dockerfile was ``2.0.1`` before, and you implement
+If the version number was ``2.0.1`` before, and you implement
 a bug fix to the image, the new image will be ``2.0.2``. The output image from
 Docker Hub will still be ``2.0`` however. If a new feature is introduced, the
-new version tagged and in the Dockerfile will be ``2.1``.
+new version tagged will be ``2.1``.
 
-We don't care about bug fix version numbers here, as RTD will only ever have one
+We don't care about bug fix version numbers here, as Read the Docs will only ever have one
 main ``2.x`` image at a time. There is no need for us to run multiple bug fix
 versions at the same time.
 

Dockerfile

@@ -1,7 +1,6 @@
 # Read the Docs - Environment base
 FROM ubuntu:18.04
 LABEL mantainer="Read the Docs <[email protected]>"
-LABEL version="5.0.0"
 
 ENV DEBIAN_FRONTEND noninteractive
 ENV APPDIR /app