Add debian docker image, drop helm2, upgrade packages. (#1956)

- Add debian image based on `stable-slim`, desire for this is largely
  around my use case using Azure DevOps which makes it challenging to
  use images which are not glibc based.
- Drop support for helm2 in the docker images. This is a tricky one but
  given that I was having errors during the docker build for helm2 and
  the fact that it has been EoL for a long time now made me think that
  this was the correct move.
- As a "while I'm in here" I've upgraded kubectl and helm. I've popped
  on the most current patch of the last release (v1.20.3) to give a
  slightly broader support for different Kubernetes versions.
- Reworked CI to support pushing a debian and alpine base, and dropped
  support for the helm2 versions.

Dockerfile.debian

@@ -0,0 +1,70 @@
+FROM golang:1.16.7 as builder
+
+RUN apt-get update \
+ && apt-get install --no-install-recommends -y \
+    build-essential \
+    git \
+ && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /workspace/helmfile
+COPY . /workspace/helmfile
+
+RUN make static-linux
+
+# -----------------------------------------------------------------------------
+
+FROM debian:stable-slim
+
+RUN apt-get update \ 
+ && apt-get install -y --no-install-recommends \
+    ca-certificates  \
+    git \
+    bash \
+    curl \
+    jq \
+    wget \
+ && rm -rf /var/lib/apt/lists/* 
+
+ARG HELM_VERSION="v3.6.3"
+ARG HELM_SHA256="07c100849925623dc1913209cd1a30f0a9b80a5b4d6ff2153c609d11b043e262"
+ARG HELM_LOCATION="https://get.helm.sh"
+ARG HELM_FILENAME="helm-${HELM_VERSION}-linux-amd64.tar.gz"
+
+RUN set -x && \
+    wget "${HELM_LOCATION}/${HELM_FILENAME}" && \
+    echo "Verifying ${HELM_FILENAME}..." && \
+    sha256sum "${HELM_FILENAME}" | grep -q "${HELM_SHA256}" && \
+    echo "Extracting ${HELM_FILENAME}..." && \
+    tar zxvf "${HELM_FILENAME}" && mv /linux-amd64/helm /usr/local/bin/ && \
+    rm ${HELM_FILENAME} && rm -r /linux-amd64
+
+# using the install documentation found at https://kubernetes.io/docs/tasks/tools/install-kubectl/
+# for now but in a future version of alpine (in the testing version at the time of writing)
+# we should be able to install using apk add.
+# the sha256 sum can be found at https://storage.googleapis.com/kubernetes-release/release/${KUBECTL_VERSION}/bin/linux/amd64/kubectl.sha256
+# maybe a good idea to automate in the future?
+ENV KUBECTL_VERSION="v1.21.4"
+ENV KUBECTL_SHA256="9410572396fb31e49d088f9816beaebad7420c7686697578691be1651d3bf85a"
+RUN set -x && \
+    wget "https://storage.googleapis.com/kubernetes-release/release/${KUBECTL_VERSION}/bin/linux/amd64/kubectl" && \
+    sha256sum kubectl | grep ${KUBECTL_SHA256} && \
+    chmod +x kubectl && \
+    mv kubectl /usr/local/bin/kubectl
+
+ENV KUSTOMIZE_VERSION="v3.8.8"
+ENV KUSTOMIZE_SHA256="175938206f23956ec18dac3da0816ea5b5b485a8493a839da278faac82e3c303"
+RUN set -x && \
+    curl --retry 5 --retry-connrefused -LO https://github.com/kubernetes-sigs/kustomize/releases/download/kustomize/${KUSTOMIZE_VERSION}/kustomize_${KUSTOMIZE_VERSION}_linux_amd64.tar.gz && \
+    sha256sum kustomize_${KUSTOMIZE_VERSION}_linux_amd64.tar.gz | grep ${KUSTOMIZE_SHA256} && \
+    tar zxf kustomize_${KUSTOMIZE_VERSION}_linux_amd64.tar.gz && \
+    rm kustomize_${KUSTOMIZE_VERSION}_linux_amd64.tar.gz && \
+    mv kustomize /usr/local/bin/kustomize
+
+RUN helm plugin install https://github.com/databus23/helm-diff --version v3.1.3 && \
+    helm plugin install https://github.com/jkroepke/helm-secrets --version v3.5.0 && \
+    helm plugin install https://github.com/hypnoglow/helm-s3.git --version v0.10.0 && \
+    helm plugin install https://github.com/aslafy-z/helm-git.git --version v0.10.0
+
+COPY --from=builder /workspace/helmfile/dist/helmfile_linux_amd64 /usr/local/bin/helmfile
+
+CMD ["/usr/local/bin/helmfile"]