Building the helm container fails

The build of the helm container fails at [1] because the docs folder have been removed from the upstream helm repo [2].

We should either pin the helm to an earlier released version that still has the docs, or copy the helm charts from the new location.

[1] https://gerrit.akraino.org/r/gitweb?p=validation.git;a=blob;f=docker/helm/Dockerfile#l42

[2] https://github.com/helm/helm/commit/4f6d002d6c07d379508b5bfe53924753a3b1d4da