ci(build): new docker image with required helm tools

We need the `helm` tool itself and the helm plugin
[`cm-push`](https://github.com/chartmuseum/helm-push) to upload to
harbor chartmuseum.

.gitlab-ci.yml

@@ -16,7 +16,10 @@ stages:
 .docker-matrix:
   parallel:
     matrix:
-      - IMAGE_NAME: [commitlint, semantic-release-gitlab]
+      - IMAGE_NAME:
+          - commitlint
+          - helm
+          - semantic-release-gitlab
 
 ###############################################################################
 # `lint` stage: `commitlint`

Dockerfile.helm

+FROM alpine:latest as INSTALLER
+
+LABEL maintainer="eole@ac-dijon.fr"
+
+RUN apk add --no-cache \
+    ca-certificates \
+    curl \
+    git \
+    openssl
+
+RUN curl -o get-helm-3 https://raw.githubusercontent.com/helm/helm/main/scripts/get-helm-3
+RUN sh ./get-helm-3
+RUN helm plugin install https://github.com/chartmuseum/helm-push
+
+FROM alpine:latest
+
+LABEL maintainer="eole@ac-dijon.fr"
+
+COPY --from=INSTALLER /usr/local/bin/helm /usr/local/bin/helm
+COPY --from=INSTALLER /root/.local/share/helm/ /root/.local/share/helm