Update charts-build-base image to Helm 3

Updates charts-build-base image to use Helm 3.4.0. This is expected to
be compatible with the existing version of kubectl in use (1.16.4) given
the version skew docs: https://helm.sh/docs/topics/version_skew

Related to https://gitlab.com/gitlab-org/charts/gitlab/-/issues/2350

Dockerfile.gitlab-charts-build-base-helm-3

@@ -0,0 +1,29 @@
+FROM ruby:2.6.5-alpine
+MAINTAINER GitLab Distribution Team
+
+ENV HELM_VERSION=3.4.0
+ENV HELM_URL=https://get.helm.sh/helm-v${HELM_VERSION}-linux-amd64.tar.gz
+ENV HELM_HOME=/root/.helm
+
+# kubectl (possibly in gcloud?)
+ENV KUBECTL_VERSION=1.16.4
+ENV KUBECTL_URL=https://storage.googleapis.com/kubernetes-release/release/v${KUBECTL_VERSION}/bin/linux/amd64/kubectl
+
+# Install dependencies
+RUN apk --no-cache add -U openssl curl tar gzip bash ca-certificates git python2\
+  && mkdir -p /opt
+
+# Install kubectl
+RUN curl -L -o /usr/local/bin/kubectl ${KUBECTL_URL} \
+  && chmod +x /usr/local/bin/kubectl \
+  && kubectl version --client
+
+# Install Helm
+RUN wget -q -O - ${HELM_URL} | tar zxf - \
+    && mv linux-amd64/helm /usr/bin/ \
+    && chmod +x /usr/bin/helm \
+    && helm version --client
+
+#Install kubeval
+RUN mkdir -p $HELM_HOME/plugins && \
+    helm plugin install https://github.com/instrumenta/helm-kubeval