[helm-tester] stop using deprecated onbuild images

docker onbuild have been deprecated in docker-library/official-images#2076 and aren't part of python docker images build since docker-library/python#314.

helpers/helm-tester/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.6-onbuild
+FROM python:3.7
 
 ENV HELM_VERSION=2.14.0
 
@@ -8,3 +8,8 @@ RUN wget https://storage.googleapis.com/kubernetes-helm/helm-v${HELM_VERSION}-li
     rm -rf linux-amd64 && \
     HOME=/ helm init --client-only && \
     chmod 777 -R /.helm
+
+COPY requirements.txt /usr/src/app/
+RUN pip install --no-cache-dir -r requirements.txt
+
+COPY . /usr/src/app