fix version string generated in docker builds (#2566)

Dockerhub changes the executable bit on `hooks/build`, which leads to
the repository being dirty during the build which messes with the
version string created by setuptools-scm. This commit changes the
executable bit so that the docker build does not make the repo dirty.

It also adds a `git reset --hard` to ensure the repo is clean even if
the build process dirties the repo in a different way in the future.

Dockerfile

@@ -28,6 +28,9 @@ ADD . /code
 
 WORKDIR /code
 
+# force repository to be clean so the version string is right
+RUN git reset --hard
+
 # Using "test" optional to include test dependencies in built docker-image
 RUN pip install .[test] && \
     apt-get purge -y --auto-remove apt-utils gcc libc6-dev libc-dev libssl-dev